SUGAR SERIES
Vol.
Vol.
Vol.
Vol.
1.
2.
3.
4.
Standard Fabrication Practices for Cane Sugar Mills (Delden)
Manufacture and Refining of Raw Cane Sugar (Baikow)
By-Products of the Cane Sugar Industry (Paturau)
Unit Operations in Cane Sugar Production (Payne)
sugar series, 4
unit operations
in cane sugar
production
JOHN HOWARD PAYNE
ELSEVIER SCIENTIFIC PUBLISHING COMPANY
Amsterdam — O x f o r d — N e w Y o r k
1982
ELSEVIER SCIENTIFIC PUBLISHING COMPANY
Molenwerf 1 ,
P.O. Box 2 1 1 , 1000 A E Amsterdam, The Netherlands
Distributors for the United States and Canada:
E L S E V I E R SCIENCE P U B L I S H I N G C O M P A N Y INC.
5 2 , Vanderbilt Avenue
New York, N.Y. 10017
Library
of C o n g r e s s
Cataloging
in P u b l i c a t i o n
Data
Payne, John Howard, 1906Unit operations in cane sugar production.
(Sugar series ; k)
Bibliography: p.
Includes index.
1. Sugar—Manufacture and refining. I. Title.
II. Series.
f
TP37T.P38 1982
661* . 122
82-11373
ISBN 0-UUU-U210U-1
ISBN 0-444-42104-1 (Vol. 4)
ISBN 0-444-41897-0 (Series)
© Elsevier Scientific Publishing Company, 1982
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher,
Elsevier Scientific Publishing Company, P.O. Box 3 3 0 , Amsterdam, The Netherlands
Printed in The Netherlands
ν
PREFACE
This compilation is intended as a guide for the efficient performance
of the several unit operations obtaining in a factory processing sugar
cane. It is not a textbook, engineering handbook, or equipment operating
manual. The purpose is only to present in simple form the basic principles
involved and some of the means of reaching optimum results.
The reader is assumed to be grounded in the fundamental chemistry,
physics, engineering, biology, economics and common sense involved in
sugar factory routine. He is also assumed to have had a reasonable length
of experience in the industry.
These practices are designed for an Hawaiian type of raw material.
That is, two-year cane, rake harvested under both wet and dry conditions,
requiring the use of wet cleaning plants. The product is a sugar averaging
99 pol, designed for delivery to a refinery.
Although some of the Hawaiian conditions are not common, most of
the practices are applicable to all cane sugar areas.
The author salutes with respect his predecessors who developed the
vast reservoir of knowledge which forms the basis of present technology.
He thanks his contemporaries who have inspired as well as contributed.
He greets the upcoming with the admonition that if they do not improve
on this, we both have failed.
vi
ACKNOWLEDGMENTS
Acknowledgment goes to Amfac Inc. for initially suggesting the writing
and close cooperation in the period of its development, to the Hawaiian
Sugar Planters' Association for permission to use some unpublished
material and to John W . Herkes who assisted in the preparation of the
chapter in Equipment Maintenance. The comments of Hideo Idehara and
J. R. Albert-Thenet are appreciated.
Drawings are the work of James
Kawamura.
Specific recognition is gratefully extended to Misue Okino
Sakamoto who assisted in editing and proofreading the manuscript and did
much of the detailed assembly work.
vii
1
When Omer smote 'is bloomin
lyre,
,
He'd eard men sing by land an' sea;
1
An -what he thought 'e might
require,
Έ went an' took - the same as me.
Rudyard
Kipling
Chapter 1
FACTORY CONTROL
The monitoring, measuring, sampling and analytical procedures
necessary for operational control and material accounting are outlined in
the Schedule for Measuring, Sampling and Analysis at the end of this
chapter. These are considered to be the minimum that will provide
adequate information for all concerned in the factory operation. It is
assumed that automatic devices are used wherever practicable.
The laboratory load is reduced to a level where only one analyst is used
per shift, exclusive of the analytical work necessary for field distribution
and agricultural control. No provision is included for special analytical
work of an experimental nature.
The procedures are those of The Official Methods of the Hawaiian
1
Sugar Technologists.
PROCEDURES
Field Cane Received
Measuring
Weigh transport units at
tolerance of ± 0.25%.
the
factory on a beam scale with a
Calibrate the scale annually, using known weights.
Check tares each shift. No accumulation of dirt on the platform and
dirt and water in the scale pit is permitted.
Report the weight of cane received as the difference between the
weights of the transport unit before and after discharging the cane. This
weight is reported with the field area designation and the transport unit
number.
Sampling
Core sample the cane on the transport.
Equipment. A horizontal core sampling machine is used. The coring
tube of 25 cm in diameter and revolves at 100 rpm. The tube enters 2 m
into the cane through an opening provided in the transport approximately
midway vertically in the load. Upon retraction, the sample in the tube is
discharged into a conveyor leading to a 25-cm Rietz Prebreaker which
chops the sample and discharges it into a revolving disc subsampler. This
drops a portion into a subsample tray and discharges the remainder onto a
disposal conveyor.
2
Procedure. The coring tube must enter the full distance into the load
and the entire contents of the tube must go through the Prebreaker.
The subsampler should
approximately 250 g (0.5 lb).
be
adjusted
to
give
a
subsample
of
The entire subsample is placed in an airtight plastic container.
Four cores from the sample area are composited in the
container and constitute one sample for analysis.
same
Frequency. The number of cores taken is based upon the
requirement to obtain a number of samples for analysis equal to the
square root of the number of deliveries per unit area per week. Four
cores are composited to make one sample for analysis in all cases where
the number of deliveries exceeds four. The number of cores and samples
needed is derived from the following table:
No. of loads
Cores
Samples for analysis
1
2
3
4
5-16
17-30
31-42
43-56
57-72
73-90
91-110
111-132
133-156
157-182
183-210
1
2
3
4
5-16
20
24
28
32
36
40
44
48
52
56
1
2
3
4
4
5
6
7
8
9
10
11
12
13
13
900
120
30
Analysis
Disintegrator Method for Fiber.
Extract with the following variations:
Pol and Refractometer Solids %
1. The frozen sample is placed in a tared 12-gauge polyvinyl
chloride (pvc) bag and weighed. The sample is broken up by beating with a
mallet on the total contents of the bag (unless over 1000 g) are added
directly to the disintegrator. The sample is not allowed to thaw prior to
disintegration. With this precaution, mercuric chloride and sodium
bicarbonate are not necessary.
2.
Pol is run on each sample.
3. Fiber is determined only on the basis of the square root of the
number of samples, making sure that at least one fiber analysis is made on
each area each day.
3
Prepared Cane
Measuring
Weigh prepared cane on a belt weighing device, with a precision of
± 2.0%, just ahead of the extracting unit. In diffuser installations, this is
immediately following the Fiberizer.
Sampling
In the absence of a chute-gate time-cycle automatic sampling
device, grab sampling is used. Samples are taken over a 5-minute period
and held in a closed container until analyzed. No preservative is used, and
the sample must be analyzed within 2 hours.
Analysis
Disintegrator Method for Fiber, Pol and Refractometer Solids,
except that no sodium bicarbonate and mercuric chloride are used as
preservatives.
Stalk Cane
Sampling
1. Cut and pull two stools of cane at benchmark locations in the
area (not exceeding 12 ha).
2. Remove trash and weigh. Sample should include all the live
stalks in the stools. A t this point, data on pest damage and cane condition
may also be recorded.
3. Reduce the cane in the Prebreaker at the core sampling station,
or in an ensilage cutter.
4.
Subsample the reduced material by quartering to approximately
500 g.
5. Place the subsample in an airtight container and preserve in a
freezer until analyzed.
Analysis
Disintegrator Method for Fiber, Pol and Refractometer Solids %
Extract. No preservatives are used.
Bagasse
Measuring
Weigh on a belt weighing device, with a precision of ± 2.0%.
Sampling
Sample in the feed chute to the conveyor or to the pneumatic
feeder. The sampling device consists of a time-cycle actuated, hydraulic
slide gate which allows bagasse to drop into a container.
The
recommended timing is every 30 minutes, giving eight openings in a 4-hour
period. The total amount collected should approximate 25 liters. The
sample is taken to the laboratory and quartered to provide two 100 g
samples.
4
Analysis
Moisture
Oven Method or Moisture Teller Method.
Fiber
Disintegrator Method. No preservatives are used.
Pol
Disintegrator Method.
First Expressed Juice
Sampling
Collect separate samples from the crusher and the discharge roll of
the first mill by hand, or by a continuous sampler.
Analysis
Crusher sample for Refractometer Solids and Pol. Purity of First
Expressed Juice is calculated from these results.
First mill sample for Refractometer Solids. This figure is reported
for First Expressed Juice since it is less affected by extraneous water
carried from the cleaning plant.
Determine pH of the first mill sample.
Absolute Juice
Calculated from First Expressed Juice
Pol, Refractometer Solids, Purity and Absolute Juice Factor.
Analyzed as Juice in Prepared Cane
Pol, Refractometer Solids and Purity.
Last Expressed Juice
Sampling
Sample by hand from the last two rolls of the tandem, or from the
dewatering device of the diffuser.
Analysis
Pol and Refractometer Solids.
Press Return
Measuring *
Measure volume by means of a proportional weir at the point at
which the press return enters the diffuser.
Sampling
Sample by hand from the press return weir box on top of the diffuser.
5
Analysis
Pol, Refractometer Solids and Insoluble Solids.
Mixed Juice
Measuring
Weigh on automatic beam scales with a precision of ± 0.1%. In
order to maintain this precision, the rate of flow of juice into the scale
tank must be uniform to insure a constant overshoot, i.e., the quantity of
juice flowing into the tank after the scale is balanced and before the valve
shuts off. Inlet and discharge valves are interlocked so that both will not
be open at the same time. The scales are equipped with a recorder
registering tank dumps.
Tare is taken at least once a shift and the scales are calibrated
annually and after any major maintenance.
Sampling
Sample automatically and proportional to the juice flow. In cases
where hot diffuser juice is weighed, the sample must be protected from
evaporation.
Analysis
Pol, Refractometer Solids, Insoluble Solids and pH.
Filtrate
Sampling
Collect a sample from the filtrate return by hand.
Analysis
Pol, Refractometer Solids and pH.
Clarified Juice
Sampling
Collect a sample from the evaporator supply tank by hand.
Analysis
Pol, Refractometer Solids and pH.
Filter Cake
Measuring and Sampling
Cut measured areas of filter cake from two spots on the filter drum,
just before the discharge blade. Weigh immediately.
From the total filter area and the number of revolutions, calculate
the total weight of cake discharged.
Use the weighed samples for analysis.
Analysis
Fiber, Moisture and Pol.
6
Syrup
Sampling
Sample syrup continuously by collecting a drip sample from
discharge of the syrup pump.
the
Analysis
Pol, Refractometer Solids and pH.
Massecuite
Measuring
Calibrate each pan for volume at various striking levels. Record the
volume for each strike and estimate the weight from the calculated
density.
Sampling
Collect a sample by hand from the flow of massecuite from the pan.
Sample at intervals during the steady flow period of discharge.
Analysis
Pol and Refractometer Solids. Calculate Crystal content.
Molasses, A , Β and C
Sampling
A - and B-molasses:
Collect sample by hand from the centrifugal discharge trough as
flow enters the pump feed tank. The sample should be taken at about the
mid-point of centrifuging.
C-molasses:
Two samples are obtained, before and after the crystallizer.
1. Filter by pressure a portion of the C-massecuite
obtained when the strike is dropped into the crystallizer.
sample
2. Filter by pressure a sample of the massecuite entering the
heater after the crystallizer.
Analysis
Pol and Refractometer Solids.
Molasses, Final
Measuring
Measure molasses in the storage at the factory with a Pneumercator
with a precision of ± 0.5%.
Calibration of Pneumercator:
The average net area of tank is determined from circumferential
measurements and the weight is calculated for a standard pressure of
2
2
0.71 g / c m (10 lb/in. ). The vertical height of the corresponding reading
7
on the manometer scale is then determined using a special calibrating jig.
It is compared with the correct height of a column of mercury at this
pressure to determine percent error. The manometer is then adjusted if
necessary to bring the actual height into agreement with the correct
height.
Official weights are obtained from the terminal where molasses
deliveries are weighed on beam scales with a precision of ± 0.1%.
Sampling
At the factory, sampling is by means of a continuous drip sample
from the discharge of the pump handling the molasses to storage.
At the terminal, sampling is conducted on each delivery.
Analysis
Samples collected
Refractometer Solids.
at
the
factory
are
analyzed
for
Pol and
A weekly composite of the factory molasses is analyzed for Pol,
Refractometer Solids, Sucrose, Reducing Substances and Ash, Carbonate.
Purity Expected is calculated.
The samples from the terminal are composited weekly and analyzed
for Brix 1-1 dilution.
Sugar - Remelt
Sampling
Sample continuously by collecting a drip sample from the discharge
of the pump delivering the remelt to storage on the pan floor.
Analysis
Pol and Refractometer Solids.
Sugar - Commercial
Measuring
Sugar is weighed at the terminal on beam scales with a precision of
± 0.1%.
Sampling
Factory:
Each strike is sampled from the
Composite for analysis on an 8-hour basis.
belt after
the
centrifugals.
Terminal:
Each delivery is sampled automatically by a bucket sampler from
the belt conveyor to the storage area.
Analysis
Factory and Terminal samples are analyzed for Pol and Moisture.
8
Condensate
Sampling and Analysis
Individual sources of condensate, evaporator cells, vacuum pans and
juice heaters are monitored continuously by electrical conductivity.
Response to high conductivity is to divert flow from boiler feedwater
supply to house hot water supply - either automatically or manually. In the
meantime, a sample should be collected and checks made with alpha
naphthol to determine if high reading is caused by sugar carryover. A high
reading may be caused by too low pH of clarified juice in the case of
evaporator condensate.
Feedwater
Sampling and Analysis
Feedwater is monitored continuously by electrical
High conductivity water is discarded.
conductivity.
Boiler Water
Sampling
A sample is collected mannually every 8 hours from a sample line
fitted with a condenser assembly.
Analysis
Alkalinity, pH, Phosphate Sulphite and Total Dissolved Solids.
For high pressure
desirable.
boilers,
Silica and Dissolved Oxygen are also
Instead of using Hawaiian Sugar Technologists Methods, standard
procedures of companies specializing in boiler water control may be used.
Condenser Water and Drains
Sampling
Condenser water from evaporator and pans,
collection areas, are sampled manually once an hour.
and
drains
from
Analysis
Colorimetric method for sugar.
A suggested Schedule for Measuring, Sampling and Analysis is given in
the following pages.
Examples of Daily, Weekly and Recovery and Loss reports are also
shown.
REFERENCES
1
Payne, John H . , (ed.) Sugar Cane Factory Analytical Control, The
Official Methods of the Hawaiian Sugar Technologists,
Elsevier,
Amsterdam, 1968 Revised Edition.
9
SCHEDULE FOR MEASURING, SAMPLING AND ANALYSIS
Stream
Cane
Field
Prepared
Stalk
Bagasse
Final
Juice
First Expr.
Absolute
Last Expr.
Mixed
Filtrate
Clarified
Filter Cake
Measurement/Method
Wt/Beam scale
Wt/Belt scale
Core sampler
Intermittent
Grab
Wt/Belt scale
Intermittent
Wt/Beam scale
Prep, cane or calc.
Grab
Cont.(grab for insol.sol.)
Grab
Grab
Wt/calculated
Grab
Grab
Syrup
Massecuite
A, B, C
Sampling Method
Continuous
Pan volume
Grab
Molasses
A , B, C
Final (Factory) Pneumercator
Final (Terminal) Wt/Terminal scale
Grab
Continuous
Grab
Re melt
Grab
Sugar
Factory
Terminal
Water
Condensate
Boiler Feed
Boiler Water
Drains
Wt/Terminal scale
Grab
Continuous
Monitor
Monitor
Grab
Grab
10
Schedule for Measuring, Sampling and Analysis (continued)
Analysis
Analytical
Method
1/24 hr
Composite
4 cores
Composite
1/4 hr
Daily/area
Ref.Sol.,
Pol, Fiber
Ref. Sol.,
Pol, Fiber
Ref. Sol.,
Pol, Fiber
HST Pol
Ratio
HST Pol
Ratio
HST Pol
Ratio
Bagasse
Final
1/1 hr
Composite
Ref. Sol,
Pol, Moist.
HST
Juice
First Expr.
1/8 hr
1/8 hr
HST
Prep, cane
or calc.
1/8 hr
1/4 hr
1/4 hr
Ref. Sol.,
Pol, pH
Prep, cane
or calc.
Ref. Sol., Pol
Ref. Sol., Pol,
pH
Insol. Sol.
Ref. Sol.,
Pol, pH
Ref. Sol.,
Pol, pH
HST
Stream
Cane
Field
Prepared
Stalk
Absolute
Last Expr.
Mixed
Sampling
Frequency
Γ
\J Trucks/area/
day
1/1 hr
1/4 hr
Analysis
Frequency
1/8 hr
1/4 hr
HST
HST
HST
Filtrate
1/24 hr
Composite
1/24 hr
1/24 hr
Clarified
1/8 hr
1/8 hr
Filter Cake
1/8 hr
1/8 hr
1/8 hr
Composite
1/24 hr
Pol
Fiber, Moist.
HST
HST
Syrup
1/8 hr
Composite
1/8 hr
Ref. Sol.,
Pol, pH
HST
Massecuite
A , B, C
Each strike
Each strike
Ref. Sol., Pol
HST
Remelt
1/8 hr
1/8 hr
Ref. Sol., Pol
HST
Each strike
Each strike
Ref. Sol., Pol
HST
1/8 hr
Composite
1/8 hr
Composite
weekly
Ref. Sol., Pol
HST
Ref. Sol.,
Pol, Sue,
Red. Sugar,
Ash, Brix 1-1
HST
Molasses
A , B, C
Final,
Factory
Terminal
Each truck
HST
HST
11
Schedule for Measuring, Sampling and Analysis (continued)
Stream
Sugar
Factory
Terminal
Sampling
Frequency
Analysis
Frequency
Each strike
Composite
1/8 hr
Composite
weekly
Pol, Moist.
HST
Pol, Moist.
HST
Monitor
Monitor
1/8 hr
Conductivity
Conductivity
pH, Alk.,
Phos. Suif.,
TDS
Sugar
Cond.
Cond.
Boiler
Control
Each truck
Water
Condensate
Boiler feed
Boiler Water 1/8 hr
Drains
1/24 hr
1/24 hr
Analysis
Analytical
Method
HST
12
DAILY
FACTORY
REPORT
AMFAC FORM 1045B 2/71
Data & Tim* Started
[Data & Time E n d a d ( S I G N E D
PERFORMANCE
PREPARED CANE
NET lAVAIL.
DELAYS
(HOURS)
PRODUCTION
TIME
EXTRACT.
BOILING
POWER
EXTRACTING
AVAILABLE
PLANT
HOUSE
[GENERATION
TIME
TIME
SOLIDS REF.
FIELD CANE
REC
IPREPARED CANE
[STALK
FILTER
CANE
CAKE
1ST E X P . JUICE
[LAST E X P . J U I C E
ICLARIFIED
IMIXED
JUICE
JUICE
[FINAL MOLASSES
DATE SAMPLED
FIELD
NUMBER
Fig.
FIELD CANE RECEIVED
1-1. Daily factory report.
PREPARED CANE
13
WEEKLY FACTORY
AM F AC FORM 1613 11/70
REPORT
FACTORY
FIELD
CANE
REPORT NO.
WEEK ENDING
SOLIDS
REF.
PURITY
SIGNED
RECEIVED
TONS
PREPARED
TONS POL
POL
%
FIBER
%
CANE
TONS
TONS POL
SOLIDS
REF.
POL
%
FIBER
%
This Week
To Date
EXTRACTING
Extraction
CANE
DILUTION %
Abs. Juka Prep.Cane|
Fiber
This Week
TONS PER HOUR
E
DAYS
HOURS
ieldCane Cetondar)
Fiber p>rep.Cane
HOURS
LOST
%
AREA
HARV.
V A R I E T I ES
%
%
%
I
To Date
BAGASSE
DELAYS
TONS
POL
%
TONS POL
Moisture
%
FIBER
%
NO
CANE
Acres
EXTRACTING
TIME
(HOURS)
EXTN.
Cleaner
PLANT.
SOILING POWER
GEN.
HOUSE
HOURS Effciency
AVAIL. I
%
MISC.
This Week
To Date
MIXED
CLARIFICATION
AGENTS
JUICE
GROSS TONS
NET TONS
TONS
SOLIDS REF.
SOLIDS
REF.
TONS POL
POL
%
PURITY
Insoluble
Solids %
LIME
TONS
MgO
TONS
Soda Ash
TONS
This Week
To Date
JUICES
FIRST EXPRESSED
iofdsRef
POL %
LAST EXPRESSED
IPURITY Saids Ref. POL %
CLARIFIED
PURITY Saids Ref POL %
FILTERED
PURITY Sofds Ref PURITY
ABSOLUTE (PREPARED CANE)
Sofds Ref. P O L *
PURITY FACTOR
Γ
I
This Week
I
To Dete
SYRUP
pH
SOLIDS
REF.
POL %
First
PURITY
FILTER
Mixed J.
Liming
Evap.
Supply
SYRUP
CAKE
TONS
TONS POL
POL%
Moisture FIBER % X PREP.
%
SOLIDS
CANE
This Week
I
To Date
STRIKE S
FUEL
A
Β
MASSECUITE
eu. ft./
t. sua.
Sofds Ref PURITY
LOW GRADE
MASSECUITE
MOL.
PURITY Sofds Ref PURITY
J
?m
This Week
MOL.
PURITY
MASSECUI ΓΕ
eu. HJ
Sofds Ref. PURITY
MOL.
PURITY
Remelt
Barrels
Purity
v#n
Π
t
To Date
F I N A L M O L A S S ES
TONS
TONS
SOLIDS REF.
TONS SUCROSE
SOLIDS
REF.
TONS POL
POL %
Sucrose
%
Per 100 Solids Ref.
Sucrose Pol Pur
RJSJAah
Purity tfeleeded) R.S.
ASH
HSJ
Cond.
This Week
To Date
SUGAR
MANUFACTURED
TONS COM'L
TONS 96 DA
FINAL MOLASSES T O TERMIN AL
This Week
Previously
To Date
Fig.
1-2. Weekly factory report.
Moisture DET*N
%
FACTOR
TONS
BRIX
1 - 1
TONS 85 BRIX
Purity
Above
Expected
14
RECOVERY AND LOSSES REPORT
STOCK
O N
Solids
Ref %
H A N D
Purity
Pol %
C u Ft
W t Per
C u Ft
X X X
X X X
Tons
Solids R e f
Tons
Tons Pol
1 Juice
2 Syrup
3 Molasses
4 Remelt
5 Massecuite
6
7
8 Totals a n d A v e r a g e s
RECOVERY
O N
STOCK
9 Pol
m
i(»- )
Tons
Tons
Solids R e f
Solids R e f
%
Pol %
Purity
Solids R e f
%
Pol %
Tons
Tons
Solids R e f
Tons Pol
10 Sugar, C o m m e r c i a l
1 1 Molasses
FINAL
MOLASSES
12 W e i g h e d For C r o p / P e r i o d
13 A v a i l a b l e in Procoss This D a t e
14 Subtotal
ι
15 In Process Beginning of C r o p / P e r i o d
16 Total D u e For These
Weeks
Tons
BALANCE
ENTERED I N T O M A N U F A C T U R E :
17
Cane, N e t
18
M i x e d Juice
19
A v a i l a b l e in Process Beginning ο
Crop/Period
X X X
X X X
X X X
X X X
X X X
Tons Pol In C a n e
Out
In
X X X
Pol %
X X X
2 0 C o m m e r c i a l Sugar P r o d u c e d
21 A v a i l a b l e in Process This D a t e
22 Loss in Manufacture;
23 Totals
LOSSES
X X X
X X X
X X X
X X X
X X X
X X X
X X X
X X X
X X X
X X X
Tons
Pol %
Tons Pol in Mixed Juice
Out
In
X X X
X X X
X X X
X X X
X X X
X X X
X X X
X X X
X X X
X X X
Total
Tons Pol
7.
24 Bagasse
%
Boiling House
Tons Pol
X X X
X X X
25
26
27
28
29
Filter C a k e
Molasses
Undetermined
Sum or T o t a l
X X X
X X X
X X X
X X X
RECOVERY
3 0 Total a n d Boiling H o u s e
m
31 A v a i l a b l e , s - j - m
*(i- )
i(s-m)
_
(
I
)
)
X X X
32 A c t u a l % of C a l c u l a t e d
X X X
Period
GENERAL
To Date
33 Tons Sugar Produced a n d A v a i l a b l e
34 Tons 9 6 D A Sugar Produced a n d A v a i l a b l e
AMRAC FORM 1907 3/76
R & L Report N o .
.Factory .
For
the_
.Weeks
ending.
_I9
_Chemist
Fig.
1-3. Recovery and losses report.
15
Chapter 2
CANE CLEANING
The essential steps in effective cane cleaning of pushrake harvested
whole stalk cane are:
1.
2.
3·
4.
Thinning
Rock, gravel and sand removal
Washing
Fibrous trash removal
A schematic diagram of a typical cleaning plant is shown in Figure 2-1.
THINNING
A thin mat of cane, 2 to 3 stalks thick, is necessary to obtain good
cleaning. Thinning action is usually effected with a carding drum over
which the cane passes. The drum, fitted with tines 30 to 40 cm (12 to 16
in.) in length, is set somewhat above and beyond the carrier and turns in
the direction of cane flow at about 300 m (1000 ft) per minute peripheral
speed.
Carding is reasonably good if the cane is not presented in large bundles, some of which tend to be thrown over the top of the drum without
carding. Partial preliminary load distribution is therefore necessary.
An alternative to the carding drum is a tumbling conveyor. This is of
pipe-slat construction hanging in a catenary arc of 55°. The angle is such
that cane in some depth starts tumbling back near the top of the conveyor
and will only pass over the top when the blanket is thin enough to give a
low center of gravity. This type of action becomes less effective with
unburned cane where tops and leaves bind the cane together.
ROCK, GRAVEL AND SAND REMOVAL
Rock, gravel and sand are the most deleterious materials in rakeharvested cane. Only by an hydraulic bath can an acceptable separation
be made. Two types of equipment are suitable.
In a velocity bath, a uniform rapid flow of water carries the cane in a
flat arc through the bath to a drag-slat conveyor. Rock, gravel and sand
(also tramp iron), being of higher density, drop in a steeper arc through
the water and fall onto a conveyor in front and at right angles to the cane
conveyor and here are conveyed to a truck for disposal. The importance
of a thin blanket of cane is apparent, as a bundle of cane will raft the
heavier materials across with the cane.
In a sink-float bath a slurry of mud and water is maintained at a
density that the cane will float and the dense materials sink. Mud density
Fig.
2-1. Cane cleaner.
16
17
3
3
should average about 1200 k g / m (75 l b / f t ) . Cane fed into the bath is
floated across by some flow of slurry assisted by a revolving dunking
drum. The same kind of cane and rock conveyors as outlined previously
are used, and, as in the case of the velocity baths, a thin cane blanket is
necessary to prevent rafting of rocks on the cane.
WASHING
Washing takes place initially on a drag-slat conveyor coming out of the
hydraulic bath. The cascade principle is used with a large volume of
water added near the top of a solid deck conveyor cascading downward in
a turbulent flooding flow sweeping the soil out through a slotted deck
above the water line of the bath.
The cascade conveyor commonly has an angle of 40° and is run at a
speed averaging 50 m/min (150 ft/min).
FIBROUS TRASH REMOVAL
The quantity of fibrous trash - tops, leaves and ground trash - is
reduced by means of detrashing rolls. There are two types, Olsen and pipe
or collared rolls.
Olsen rolls are the most effective. These are constructed in banks
consisting of several pairs of rolls made up of rectangular plates welded to
a shaft. The banks are set at an angle 45° to 50° and the cane slides down
over them by gravity. On revolving countercurrent to the cane flow at
speeds in a range of 120 to 200 rpm, the corners of the plates enmesh,
drawing leafy material down through and discharging onto a belt below,
while the cane slides to a conveyor. Water jets help to push the trash
between the rolls. Banks of Olsen rolls are often separated by single pipe
rolls.
Collared rolls may be used both before and after the hydraulic gap.
Collared rolls are pipe rolls with welded circular discs which enmesh.
They are set in banks as with Olsen rolls, but at a lower angle, usually
30°. As the cane slides over the rolls, water jets force loose material
through the rolls while the cane travels over them. The rolls turn with the
flow of cane, thus propelling it. Speed is of the order of 60 to 180 rpm.
Effective trash removal requires that the cane pass over the
detrashing rolls at single stalk depth. Thus, the cane blanket must again
be thinned after cascader washing. This is done by means of combing
drums which not only spread out the cane but also distribute it to the
separate banks of detrashing rolls.
Combing drums, 4 to 10 in number, are set on an upward slope of 20°
to 30°. They are of relatively large tip diameter up to 1.8 m (6 ft) and are
fitted with enmeshing serrated rings of steel plate. Rotating in the
direction of the cane flow at progressively increasing tip speeds, they
propel the cane with a bouncing action. Spacing is such that short cane
will fall between the drums whereas long cane goes over the top of the
last drum. By arranging the chutes below, distribution to the sets of
18
detrashing rolls is made. Jets of water are used in additional washing of
the cane.
OPERATION
Cane cleaning is the most difficult of all factory operations to control
and maintain. The one major problem is that of keeping a continuous, thin
blanket flow of cane. Success with that gives good cleaning. Without it,
there is poor cleaning and lost time caused by jams and equipment failure.
The initial dumping of large loads of cane on the feeder table is the
cause of most of the problems in distribution. The only reasonably
satisfactory solution to this is one in which the cane is raked from the
feeder table to the first cane conveyor. This is best followed by double
carding, one after the first carrier and one ahead of the hydraulic bath.
Without a raking system, care must be taken in feeding from the table to
the conveyor to give partial distribution.
From the carding drum on, the key is continuous steady flow. The
necessity of keeping the clean cane conveyor full too often leads to a
stop-go operation in which the cane may be flowing only half the time.
This means that the cane blanket averages double the intended and the
cleaning efficiency suffers accordingly.
Controls are set so that any failure in the cleaner automatically stops
all equipment behind in order to prevent jams. However, the conveyor
after the hydraulic bath should always be kept running if possible, long
enough to clear the cane from the bath.
Conveyor speeds through the cleaner should be high enough to provide
a minimum depth of cane.
Water distribution should be concentrated at the points of maximum
effectiveness. These are the cascader, combing drums and detrashing
rolls. The quantity at each point must be sufficient to give a complete
flooding action, otherwise effectiveness is lost. It must also be
remembered that high pressure sprays are only effective at the point of
impact.
CANE SALVAGER
In any hydraulic bath there are always some cane stalks, usually short
lengths, that reach the bottom and are discharged on the rock and sand
conveyor. These are recovered in a cane salvager which is essentially a
small-scale velocity bath. Such a device should recover most of the cane
which might otherwise be lost.
EFFICIENCY OF CLEANING
The quantity of extraneous matter delivered to the factory with the
cane varies widely depending on harvesting conditions. With dry weather
harvesting, a good burn will eliminate most of the dry leafy material
leaving only tops and some ground trash. Soil and rock pickup will then be
19
a minimum varying with the terrain. In wet weather harvesting, with
little or no burn, most of the fibrous trash will remain and large quantities
of soil and rock will be present. Often during poor harvesting conditions
the extraneous material can exceed the quantity of cane.
Most cleaners are designed to handle cane with an average amount of
extraneous matter and will not be effective under extreme conditions. In
considering cleaner efficiency, it is only possible to look at the order of
magnitude involved and to separate irrigated regions (relatively dry) from
unirrigated regions (relatively wet). Even so it is only possible to get a
reasonable handle on fibrous trash.
1
Early data by the Experiment Station of the Hawaiian Sugar Planters
1
Association ^ showed that the cane plant after two years of growth
would have produced the following:
Average makeup of total dry weight produced in two years:
%
Tops
Leaves
Roots
Stalks
5
24
2
69
Composition at time of harvesting:
Moisture
%
Tops
Leaves
Roots
Stalks
76
34
—
70
Fiber
%
20
64
—
13
During the two years of growth, leaves fall off and become ground
trash, some of which decomposes. So at the time of harvest the leafy
matter is considerably reduced. With a good burn most of the dry leaves
burn leaving little fibrous trash except for the tops. The following figures
are indicative of various conditions.
Fibrous Trash Delivered to Factory
% of Gross Cane
Burned
Tops
Leaves
Total (av.)
Irrigated
Unburned
3.5
0.5
5.0
11.0
12
Unirrigated
Burned
Unburned
5.0
8.0
7.0
18.0
20
To these figures must be added dead cane, which is considered to be trash,
and stools (root structure).
The quantity of soil and rocks can range from a low of 1.5% under dry
conditions to over 20% for wet conditions.
20
The cleaners in general perform well in removing soil and rock, the
efficiency being in the range of 95%. Removal of fibrous trash, however,
is rarely over 50% and usually can be considered to average 40%.
Essentially no dead cane is removed. Stools, for the most part, are taken
out in the rock bath.
Removal of soil and rock is the most important function of the
cleaner, since those materials place high wear loads on the equipment.
Fibrous trash contributes only to increasing the quantity of fiber to be
processed.
With a high return value placed on fiber for fuel it can be economically
sound to return all fibrous trash to the extraction plant. In this case the
fiber should be given a secondary treatment, essentially the same as in the
rock bath of the cane cleaner, to remove soil and rock fragments.
WATER REUSE
Clean water consumption can be cut substantially by multiple use.
Extremely muddy water cleans equally with clean water except for a
small residual. Minimum clean water use could therefore be only on the
detrashing rolls, and perhaps a final rinse. That water is then collected
and used on the combing drums. Then that from the combing drums on the
cascader and finally in the hydraulic bath. Additional steps may also be
used.
Clean water requirements are considered to be about 4000 liters/
metric ton (1000 gal./short ton) clean cane per hour. This can be reduced
with more efficient application and more stages of reuse.
WASTE DISPOSAL
Rock, gravel and sand are sent to land fill disposal.
Fibrous trash may be returned to the fields but is useful to salvage for
fuel. Two methods are used. One is to truck it to an open area, spread it
out and allow air drying. The dry material is returned to the bagasse
storage area, reduced in a shredder and fed to the furnace with bagasse.
Another method is to treat the trash in a rock bath to remove rock
fragments and sand and return it to the mill for dewatering.
Waste water is allowed to settle in ponds or Hydroseparator-type
equipment, with the effluent returned to the fields and the settlings
pumped to landfill areas.
LOSSES
Losses in cane cleaning are in two categories - mechanical, loss of
cane with rocks and with fibrous trash, and sugar loss by washing out of
juice.
21
The mechanical loss can be kept at a level of 1% or below by use of a
cane salvager and by proper operation and maintenance of detrashing
equipment.
The loss of juice (pol) is dependent upon the damage inflicted on the
cane in harvesting, loading, hauling and cleaning. A single stalk of cane,
sharply cut, can be washed thoroughly with negligible pol loss. A damaged
or roughly cut stalk loses pol proportionally to the extent of the damage.
Accurate quantitative data on pol losses are extremely difficult to
establish because of the wide variation in damage and the extraordinary
difficulty in the representative sampling of cane. Furthermore, measurements made on a weight basis are of limited value since the weight of
the cane varies in each step of treatment - increasing with accumulation
of soil, decreasing with removal of soil, increasing with accumulation of
water and decreasing with loss of water and juice.
Comprehensive investigations of the magnitude of pol loss in cleaning
were made by the Sugar Technology Department of the Hawaiian Sugar
1
Planters Association on the basis of pol per unit fiber measurements.
Although there was wide variability in individual cases the average
statistically significant loss was 3.3%. Adding mechanical losses to this
brings the value to around 4%. In economic appraisals a round figure of
5% would be a reasonable magnitude.
Of course this is only the loss in cleaning. In the field operations
during harvesting, the losses are of about the same magnitude bringing the
total attributable to rake harvesting to the 10% range.
REFERENCES
1
2
Borden, Ralph, Hawaii Planters Record, 46 (1942) 191-238.
Stewart, Guy R., Assoc. Hawaiian Sugar Tech., Reports (1929)
221-230.
23
Chapter 3
MILLING
INTRODUCTION
Milling is basically an exercise in materials separation. In the simplest
concept, cane consists of a solid - fiber, and a liquid phase - juice, which
must be separated before sugar can be produced.
This is done in a milling tandem in which juice is expelled from the
fiber by successive applications of pressure as the cane passes between
pairs of rolls. The efficiency of juice separation is determined by:
Number of squeezes
Effective pressure
Degree of cell rupture
Drainage
Physical properties of fiber
The capacity of a tandem is determined by the ability of the rolls to
accept the cane presented and transport it by friction between the rolls.
This is called feedability. The capacity is governed by the quantity of
fiber since the juice alone puts no load on the rolls.
In practice, a milling plant is designed for a nominal capacity at a
nominal juice recovery. That is, the tandem should accept the quantity of
cane desired in a unit time and expel a targeted percent of the juice. The
actual results obtained depend upon how the tandem is set, how it is
operated and how it is maintained.
By pressure alone, it is impossible to expel much over 90% of the juice
from the fiber because at some point the solid and liquid phases
essentially coalesce into a mass which extrudes forward. Therefore, in
order to recover more of the juice, it is necessary to add water. The
water mixes with the juice and a certain percentage of the diulted juice is
expelled in the next pair of rolls. By repeating this process, it is possible
to recover substantially all of the juice.
A conventional milling plant comprises equipment to prepare the cane
for milling, which may be rotary knives, shredders, or combinations of
knives and shredders; and a series of four to six 3-roll mills. In some
installations a 2-roll mill, or crusher, may precede the mills. A crusher is
considered both a preparatory machine and a mill. Each mill is normally
equipped with a feeding device consisting of one or two rolls which
compress the fiber before presentation to the mill. In some cases, these
feeding devices become essentially a part of a mill resulting in 4-roll and
5-roll mills.
24
CANE PREPARATION
Efficient juice separation in a milling tandem requires good preparation of the cane, whether obtained in preparatory machines, such as
shredders, or in the mills alone. Good preparation means release of a high
percentage of juice from the cellular structure of the cane without
reducing the fiber size to such an extent that it will not feed well when
presented to the mill. An approximate measure of the release of juice is
given by a factor called Displaceability Index which gives the amount of
juice that can be replaced by water in a fixed period of time.
Preparation is best performed at the beginning of a milling tandem,
and the best equipment is a heavy-duty shredder which essentially strips
the fiber from the cane, releasing the juice, but still retaining a relatively
long fiber structure. Such preparation also gives the best feedability. A
well-designed shredder with heavy hammers (20 kg) running at a tip velocity of 6000 meters (19000 ft) per minute, should give a Displaceability
Index of close to 88%. The horsepower necessary is about 13.2 per metric
ton (12 per short ton) cane per hour.
Running at lower speeds reduces the preparation and the horsepower.
At 4500 meters (15000 ft) per minute, the Displaceability Index would fall
to around 85% and the horsepower to 9 per metric ton (8 per short ton)
cane per hour.
In general, increasing the Displaceability Index by 4% would give a 1%
increase in extraction in the same milling tandem. This is approximately
the equivalent of adding one mill to a tandem.
Lighter shredders are more effective if installed after a crusher, which
provides protection from rocks. Also, by crushing the fiber and removing
some of the juice, the power requirement is reduced. Such a shredder
operating at a tip velocity of 4500 meters (15000 ft) per minute should
give a Displaceability Index of 75% at a power requirement of 3.3 horsepower per metric ton (3 per short ton) cane per hour.
TWO-ROLL CRUSHERS
Two-roll crushers, formerly common in milling tandems, are rarely
part of new installations. They are nevertheless simple and useful
adjuncts. They serve the three functions of cane preparation, juice
extraction and improved feeding for the next mill.
Crushers are commonly the first unit of the milling train, although
heavy shredders are often installed ahead of crushers. A well-operated
crusher should give 50% juice extraction. By a combination of cane
preparation, blanket compression and dewatering of the fiber, the first
mill following (without maceration) should give an additional 27% juice
extraction, giving a total of 77% by dry crushing.
Crushers with coarse 7.5 cm (3 in.) circumferential grooving have good
feedability and good drainage. Hard-faced, Krajewski-type corrugations
perform a good job of rock crushing, which is an important function in
protecting shredder hammers and subsequent mills.
25
MILLS
The three rolls of a conventional mill are arranged in a triangle so that
the fiber is squeezed twice between the top roll and the feed roll and the
top roll and the discharge roll. The rolls have cast iron, grooved shells
mounted on steel shafts. Fiber passing between the top and feed roll is
conducted over a turner plate to the discharge roll. The rolls are pinion
driven from the top roll which is driven at a speed of 3 to 6 rpm by a
turbine through a gear reduction system.
The feed and discharge rolls are fixed, while the top roll is free to
move up and down by means of an hydraulic pressure system.
Cane is moved between mills by means of intermediate conveyors.
They are generally rake or drag-slat type which carry the fiber to a fixed
chute leading to the next mill.
FEEDING DEVICES
The capacity of a mill to handle cane is governed largely by the ability
of the rolls to accept the feed. Improved feedability can be obtained by
the use of devices which compress the fiber blanket presented to the rolls,
thus increasing frictional grab. These include:
Overfeed roll
Underfeed roll
Pressure feeder
Two-roll feeder
Overfeed Roll
A roll on top of the blanket just ahead of the mill compresses the fiber
and thus improves feeding. These rolls are usually of smaller diameter 75% that of the mill rolls and are fabricated from sheet steel fitted with
longitudinal angle iron strips. Sometimes cast iron rolls with grooving are
also used. The rolls are driven by sprocket and chain at peripheral speeds
10% or more above that of the mill rolls.
Underfeed Roll
The underfeed roll is generally preferred to the overfeed roll. It
operates beneath the blanket, compressing it against the top roll, which
improves feedability. Construction is similar to that of an overfeed roll
and drive is usually by sprocket and chain. The diameter is generally
smaller - 75% that of the mill rolls and the peripheral speed is the same or
somewhat higher than the mill rolls.
In some regions the underfeed roll has been increased to the same size
as the mill rolls, enmeshes with the feed roll and is driven by a crown
wheel off of the top roll. The result is a 4-roll mill which has good
feedability but has the disadvantage of poor drainage. At high mill
speeds, expressed juice which cannot drain over the feed roll flows over
the top roll and must be carried away from the discharged bagasse by
means of a scraper trough. With good design these mills perform well at
high grinding rates and offer a relatively easy method of increasing the
capacity of an existing tandem.
26
Pressure Feeder
A pressure feeder has two feed rolls the same size as the mill rolls set
at an angle like a crusher. The rolls discharge into a pressure chute
leading to the mill. The bagasse is thus kept compressed between the
feeder and the mill. The feeder rolls are geared together with pinions and
are driven by the mill turbine at a speed averaging 30% more than the mill
rolls.
The pressure feeder originated in Australia in order to improve the
feedability of bagasse from hot maceration baths. Since a feeder extracts
up to 40% of the juice from the fiber, it is not a feeding aid in the true
sense. In reality the device converts a 3-roll mill into a 5-roll mill.
Juice expressed by the top roll of the feeder is collected in a tray with
scraper arrangement, while juice from the bottom roll flows both foward
and backward into a collection trough.
Because of juice extraction, the 3-roll mill is presented with a drier
feed permitting the use of lower work ratios in the mill. The capacity of
the 5-roll mill is up to 25% greater than that of a 3-roll mill of the same
size, but power requirements are correspondingly increased.
Two-Roll Feeder
The 2-roll feeder is similar to a pressure feeder without a pressure
chute. This is a true feeding aid as it does not extract juice. It is chain
driven, and the bagasse discharges into a closed chute leading to the mill.
The device gives good compression and has the advantage of good drainage.
Because of lower power requirement and less cost, the 2-roll feeder is
generally preferred over pressure feeders in application to existing
installations.
GROOVING
Mill rolls are grooved to improve feedability and provide drainage.
Grooves are of three types:
Circumferential
Juice (Messchaert)
Chevron
Circumferential Grooves
Cutting grooves around the roll gives a corrugated surface of increased
area which has better gripping action because of the compression of the
fiber against the walls of the V-shaped grooves. Since the bagasse at the
bottom of the groove is not so highly compressed, a drainage channel for
juice is formed.
The surface area changes with the angle of the groove, the sharper the
angle the greater the surface.
Larger pitch increases the speed
differential between the tip and the channel. This gives a grinding action
which will open up cells. Although sharper grooves give more surface, the
effective surface is not much affected by very sharp grooves (30-35°)
27
since little pressure reaches the bottom of the channel. This is useful,
however, as better drainage results. Sharper grooves are more susceptible
to damage by rocks and tramp iron so angles less than 35° are not used.
Normally, 45° is most practicable where damage potential is severe.
Other conditions being the same, juice extraction decreases with
increase in pitch of grooving. This is the composite effect of several
factors, including reduced effective pressure, more groove wear caused by
higher differential speed and more slippage.
As preparation increases down the milling tandem, finer grooving is
used. Good standard practice on the last mill is 1.25 cm (1/2 in.) pitch.
With a good shredder, such grooving can be used throughout. With poor
preparation, 2.5 cm (1 in.) pitch is effective. In the absence of a shredder,
5 cm (2 in.) pitch is common in the beginning mills.
Juice (Messchaert) Grooves
Juice grooves are narrow channels cut deeper than the circumferential grooves to give better drainage. They are normally not over 0.6
cm (1/4 in.) wide to prevent much fiber being compressed into them, and
are of a depth sufficient to carry away the juice. This averages about 2.5
cm (1 in.). The pitch is also a function of the volume of juice and is
normally 7.5 cm (3 in.).
The juice grooves must be kept clean by scrapers. Wear causes
problems by enlarging the grooves and letting excess bagasse enter with
the juice. Narrow and shallow grooves are desirable for this reason.
Juice grooves are most effective on the feed roll where juice flow is
greater. They may also be used on the discharge roll but are not as
efficient because they reduce the effective pressure area of the roll and
increase the incidence of metal lost by fracturing of the circumferential
groove adjacent to the juice groove.
Chevron Grooves
Chevron-shaped grooves are cut lengthwise through the circumferential grooving as an aid to feeding. They are nominally cut to one half
the depth of the grooving and at a pitch of 25 cm (10 in.).
Chevron grooves are effective only on the
be detrimental on the discharge roll. Since
effective pressure surface of the rolls and
mixed juice, more efficient operation of
chevron grooves by arcing maintenance of the
feed and top rolls and would
such grooving decreases the
causes excessive bagasse in
a mill is obtained without
circumferential grooving.
OPERATION
Effective operation of a milling station involves proper setting of the
individual mills; close control of the operational variables, chief of which
are fiber feed rate, speed, hydraulic loading and imbibition; and finally,
good maintenance.
28
Mill Settings
The setting of a mill requires three measurements - the distance
between the top roll and the feed roll, the distance between the top roll
and the discharge roll and the distance between the top roll and the turner
plate. The weight of fiber passing through the mill per unit time is the
basis for calculating the proper setting. The following concepts are used
in arriving at the initial setting. In the course of operation, changes from
the initial are often necessary because of the many unknown variables
involved.
Work Opening:
The work opening is the average distance between the rolls as
measured on the common axial plane. It is calculated from the mean
diameter of the two rolls, which is the diameter half way between the tip
and bottom of the grooves. A fixed amount is added to the measured
opening to allow for the lift of the top roll when floating in operation
under the calculated fiber load.
Mill Ratio:
The mill ratio is the ratio of the feed and discharge settings. The
mill ratio adopted is based upon the calculated discharge work opening
which is determined by the fiber rate, size of the rolls, speed of the rolls
and fiber content of the discharged bagasse.
Once the discharge work opening is chosen, the feed work opening is
chosen depending upon trash content of the cane, preparation, drainage
and imbibition rate. The mill ratio should be kept at the minimum
practicable.
Nominal values for mill ratios are 2.5 for the first mill and 2.0 for
mills thereafter. With 5-roll mills, ratios as low as 1.5 are possible.
Escribed Volume:
The escribed volume is the product of the roll length, the work
opening and the roll peripheral speed.
Fiber Index:
Fiber index is the weight of fiber per unit escribed volume of
bagasse from the discharge roll of the mill. It is a measure of blanket
thickness and compression. Although it would appear that a high value of
fiber index is desirable, there is an optimum value beyond which little
change occurs for a given mill. The value increases from the first mill to
3
the last. Nominal maximum values of fiber index range from 560 k g / m
3
3
3
1
(35 l b / f t ) for a first mill to 880 k g / m (55 l b / f t ) for a fifth mill. Usual
operating values are lower.
Fractional Fiber Content:
The fractional fiber content expresses the fiber on a weight basis,
that is, the weight of fiber per unit weight of bagasse discharged. An
indication of nominal values is shown by data from Mauritius. 2
29
Fractional fiber content of
discharged bagasse
%
Crusher (2-roll)
Mill 1
2
3
4
5
0.25
0.37
0.41
0.46
0.49
0.52
3
3
3
Using values of 1530 k g / m (95.5 l b / f t ) for fiber and 1028 k g / m (64.2
3
l b / f t ) for juice, these figures are considerably lower than the maximum
values shown under Fiber Index.
Reabsorption Factor;
The reabsorption factor is a measure of the extrusion that occurs in
a mill. Extrusion begins when the speed of the mills becomes high enough
that the maximum pressure on the blanket of fiber begins to move ahead
of the line of minimum distance between the rolls. Material is then
propelled forward at a speed exceeding that of the rolls. A better term to
describe the phenomenon is forward slip. When this occurs, the volume of
the bagasse exceeds the calculated escribed volume. The increase in
volume is caused largely by an increase in the amount of juice, since it
moves forward preferentially to the fiber. Juice extraction decreases,
therefore, as the amount of forward slip increases.
The reabsorption factor is:
No-void bagasse volume
Escribed volume
The factor increases with roll speed, increases with fiber rate and
increases with a smaller discharge work opening. It decreases with the
fineness of the fiber.
In setting a mill this factor must be kept as low as possible. Because
of the varying fiber load, the actual value of the factor can only be
approximated. The optimum range for the factor, however, is 1.3 to 1.4.
Procedures:
Procedures adopted for setting mills, although based primarily on
fiber throughput, must take into consideration many other factors - most
important of which are:
Mill strength
Hydraulic loading
Roll grooving - size and condition
Cane quality - trash and soil content
Cane preparation - Displaceability Index
Drainage
Imbibition - quantity of water and temperature
The first step, however, is to calculate a tentative work opening for
30
the discharge roll. This is based on the fiber rate using a nominal Fiber
Index as discussed earlier. A mill speed is selected which will keep the
reabsorption factor at a reasonable level (below 1.5). The work opening
for the feed roll is then chosen keeping the mill ratio as low as possible.
For mills after the first, a nominal 2.0 mill ratio is standard. With a 5-roll
mill, this can be lowered to 1.5. Better cane preparation will permit a
lower mill ratio, as will more efficient feeding devices. Since feedability
is impaired by hot maceration, higher mill ratios are necessary. In
general, also, higher hydraulic loading, poor drainage and fine grooving
require higher mill ratios.
Turner Plate Settings:
Turner plate settings are the ratio between the feed work opening
and the opening at the toe of the plate. The opening at the toe of the
plate is equal to the set opening plus one-half depth of the top roll groove
plus hydraulic lift. There is only limited experimental evidence on which
to base settings, so general experience is used. If the plate is set too high,
the top roll will be lifted by fiber passing over the plate. This will
increase power, increase wear and lower extraction. Also, the mill
capacity will be decreased. If the plate is too low, there will be too high
an angle of contact with the discharge roll which can cause choking.
Experience indicates that an average fiber rate of 160 kg/m^ (10 lb/ft^)
on preliminary mills and 240 kg/m^ (15 lb/ft^) on the last mill gives good
performance. The ratio of toe of the plate to the feed opening is normally
1.8 for finely divided bagasse. With coarse bagasse, this ratio would
decrease to 1.2.
Maceration
In the conventional compound maceration system, water is distributed
laterally to the bagasse on the intermediate carrier feeding the last mill.
Last expressed juice is then returned to the carrier to the next to last mill
and so on up to the second mill. Juice from the second mill mixed with
the dry crushing juice from the crusher and first mill constitute mixed
juice.
Although bagasse fiber will absorb about 650% its weight of liquid, it is
not advantageous to use enough water to bring the liquid content to this
point on the last mill. A t about 250% water on fiber, the effect on
extracation levels off, so that steam and evaporator capacity
requirements dictate little economic advantage of using more water.
The water should be applied at as high a temperature possible without
causing a feedability problem. The fiber becomes plastic at higher
temperatures so is more easily compressed giving higher juice expression.
Higher temperature also makes the fiber slicker so it does not feed as well.
Mixing of liquid with the bagasse takes place mainly at the entrance to
the mill where the increasing pressure forces the liquid backward through
the fiber. Less lateral flow occurs, so the important consideration in
applying the maceration is to insure uniformity across the width of the
bagasse blanket.
31
Hydraulic Loading
Juice extraction increases with increase in pressure applied to the top
roll. The pressure that can be applied, however, is limited by the
mechanical strength of the mill. Also, feedability decreases at higher
pressures and power necessary increases substantially.
The optimum
pressure, therefore, is that which permits the top roll to float at the
necessary feedability.
The actual loading used on a given mill can only be determined by
experience. General standards call for a nominal pressure of 14 mt/m
(50 t / f t ) on a 2-roll crusher to 21 mt/m (75 t / f t ) on the last mill.
Speed
Other conditions, such as preparation and pressure, being the same,
juice extraction decreases with increased mill peripheral speed. The
reason for this is that extrusion (reabsorption, forward slip), meaning that
juice is expelled with the fiber, is a function of mill speed. It is desirable,
therefore, to operate at as low a speed possible, commensurate with the
necessary fiber rate. The finer the preparation, the slower the optimum
speed. This is because of better feedability. Also, there is the factor of
cell rupture in the mill which, because of closer mill settings, increases
with speed.
It is standard practice to run all mills in a tandem at close to the same
peripheral speed with a slight increase toward the last mill to improve
feedability.
General Effects
The general effects of operating conditions on milling efficiency may
be summarized as follows:
Pressure Increase
Increases juice expelled
Increases power required
Decreases feedability
Roll Speed Increase
Decreases juice expelled
Increases feedability
Cell Rupture Increase
Increases juice expelled
Increases feedability
Imbibition Increase
Increases juice recovery
Decreases feedability
Imbibition Temperature Increase
Increases juice recovery
Decreases feedability
32
Mill and Turner Plate Settings Decrease
Requires mill speed increase to maintain throughput
Increases cell rupture
Increases power required
CONTROL
The primary control figures on milling are pol, moisture and fiber in
final bagasse. These, of course, give only the total result and it is
important to know the performance of individual mills. This requires
bagasse analyses from each mill. But constant variation in the fiber load,
because of trash, renders such figures of little value unless taken on a
statistically significant basis. Therefore, they are commonly not justified
from the cost standpoint. As a result, mill settings are estimated and
then adjusted on the basis of experience.
Juice Density Curves
Useful guides to the behavior of each mill are juice density curves.
These are made by an analysis of the refractometer solids in samples of
juice from the feed roll, discharge roll and total from each mill.
Discharge roll figures are plotted giving a curve which for a properly
functioning tandem would appear as in Figure 3-1.
Typically, the crusher value is lower than the first mill because of the
extraneous water on the cane from the cleaner. The figures then fall in a
smooth curve to the last mill. Since the refractometer solids content of
the juice from the discharge roll of the last mill is about equal to that of
the juice remaining in the bagasse, the lower the figure, the better the
extraction. The actual value will depend upon the cane preparation and
maceration efficiency as well as the performance of the individual mill.
The individual values on the curve indicate the total performance up to
that point. A deviation from a smooth curve is evidence of malperformance in that area. Usually the deviation is toward making the curve
more horizontal and means that the preceding mill is to blame, either by
total performance of the mill or poor extraction at the feed roll permitting lower density juice from the discharge roll. Rarely it might also
be caused by very high extraction of the feed roll of the succeeding mill.
A break, making the curve more vertical, is sometimes encountered.
This could be caused by malfunctioning of the mill preceding the one
where the break occurs either in the total extraction or poor extraction at
the feed roll allowing low density juice from the discharge roll. It could
also be the result of low extraction from the feed roll of the mill
succeeding the break.
Juice density curves are only significant on a routine basis to show
deviations from previous curves. An individual curve, standing alone, is of
little value, mainly because of the idiosyncrasies of individual mills. Once
a curve with a break shows up, the test should be repeated to make sure
that a real condition exists.
33
Fig. 3-1. Mill juice curve.
The relative extraction of the feed roll and discharge roll can also be
calculated from the juice density figures, as an example:
Refractometer solids
Feedroll
Discharge roll
Total juice
2.0
4.0
2.0
Calculating by the rectangle method:
2.0
1.5
2.5
4.0
0.5
2.0
34
1.5/2.0 χ 100 = 75% of the juice extracted by the feed roll and 25% by the
discharge roll. Such figures would be indicative of good mill performance.
REFERENCES
1
2
Van Hengel, A . and Douwes Dekker, K., Some Notes on the Setting and
Operation of Mills, Proc. So. African Sugar Tech. Assn., 32nd
Congress, 1958, pp. 57-67.
Van Hengel, Α . , Some Additional Notes on Mill Settings, So. African
Sugar J., 1958, pp. 855-861.
35
Chapter 4
DIFFUSION*
INTRODUCTION
In the separation of juice from the fiber in sugar cane by the process
called diffusion, the juice is displaced from disintegrated cane by the
countercurrent flow of water rather than being expelled by pressure as in
milling. The unit operations involved are three:
Cane Preparation
Juice Displacement (Diffusion)
Bagasse Dewatering
CANE PREPARATION
Cane preparation for effective juice displacement requires size
reduction to give a compact permeable bed and rupture of close to 94% of
the juice storage cells. This should be brought about with a minimum of
grinding of the fiber and retaining a fiber bundle length of 10 to 15 cm
(4-6 in.). The cell tissue should be stripped from the fiber bundles
producing a mixture of shreds and pith tissue. Such preparation will yield
a compact yet permeable bed for countercurrent extraction.
The equipment most suited for this work is swing hammer shredders.
All experimental investigations have shown that good preparation is best
achieved in two steps. Shredders developed specifically for this purpose
are the Silver Buster and the Silver Fiberizer. The Buster has 20 kg
hammers rotating at about HOOrpm, working over an open grid screen
grate with anvils between the hammers. Feed control to the machine is
provided by a set of variable speed feed rolls. As distinct from
conventional shredders, where the hammers are working over a solid
ridged plate, cane is extruded through the screen so that no unshredded
stalk can pass. Size reduction to a length of 5 to 10 cm (2-4 in.) is
obtained with cell rupture up to 87%.
Secondary preparation takes place in the Fiberizer which is more like a
conventional shredder in that the hammers are working over a ridged 90°
arc segment. Here, fiber length is reduced only to a limited extent and
cell rupture is increased to 94%.
This discussion applies specifically to the Silver Ring Diffuser. However,
the general principles involved are the same for any type of diffusion
equipment.
36
JUICE DISPLACEMENT
Displacement of juice by water in the prepared cane is attained by a
countercurrent
liquid flow in an advancing front similar to the
phenomenon of saturated flow in porous media. The advancing liquid
operates like pistons through the ruptured cell modules replacing the
juice. Any macro breaks in the bed lead to piping, channelling and by-pass
of liquid from the displacement process. Any mixing in the system
reduces the effective countercurrent flow. Any pressing or squeezing will
expel juice concurrently as well as counter, causing mixing. The ideal
system, therefore, is an undisturbed bed of fiber and juice and plug type
liquid flow.
In the Silver Ring Diffuser, this mechanism is obtained with a rotating
annular bed of prepared cane through which the liquid is pumped in 18
stages (Fig. 4-1). Cane is made to form a continuous bed about 1.5 m (5 ft)
in depth, which is maintained for some 335 angular degrees. At this point,
the bed is discharged vertically by means of multiple screws. Water is
added from a distributor above the bed and several degrees ahead of the
discharge screws. The water flows through the moving bed and into a
compartment below and behind the distributor. From there, the liquid is
pumped forward to the third distributor, passes through the cane bed and
collects in the next juice compartment. Juice from bagasse dewatering
enters the distributor just forward of the water distributor and flows into
the second collecting tank. Flow continues forward to the point at which
the cane enters. Here the juice is cycled ahead as first pass juice. On
passing through the newly deposited cane, it picks up some suspended
matter which may be removed by circulating through the bed. This second
pass is called recycle, and pumping is in the direction of cane travel. The
effluent recycle juice is analogous to mixed juice from a miling tandem
and is sent to the boiling house. With large amounts of soil entering with
the cane, this recycling can cause plugging by depositing soil on top of the
bed, so should not be used.
Efficiency of juice displacement is reduced by some channelling in the
bed and by mixing at the overlapping boundaries of the stages in the bed.
This is compensated for, to a large degree, by the use of more than the
theoretical number of stages, and by application of more water. A typical
extraction pattern obtains however. This is shown in Figure 4-2 illustrating the gradients occurring in routine diffuser operation. A rapid drop
in pol takes place initially as juice is displaced. Then the decrease becomes more gradual and, finally, becomes asymptotic near the discharge
point, being "supported" by the juice returned from bagasse dewatering.
Factors which govern the forms of the gradient are:
Cane Quality
Cell Rupture
Bed Depth
Speed of Rotation
Bed Permeability
Quantity of Water
Quantity and Quality of Juice Return From Dewatering
Fig.
4-1. Diffuser flow diagram.
37
Fig.
4-2. Diffuser gradients.
38
39
Operational conditions can be adjusted to give the optimum results
consistent with the objectives. Pol extraction increases with increase in
cell rupture and quantity of water. There is an optimum bed depth and
speed of rotation, depending upon the cane rate and bed permeability.
It is basic that the capacity of the diffuser is governed by the rate of
the gravity flow of liquid through the bed. Cane quality, type of
preparation, bed depth and dewatering juice quality are the variables
affecting this flow rate.
BAGASSE DEWATERING
Following juice displacement, the bagasse must be dewatered. This
step also is the final stage in the extraction process as some pol in
unbroken cells and in the residual liquid is recovered and returned to the
diffuser. Dewatering requires some sort of expelling mechanism, and
most of the common types of machines are adaptable to such use. First
thought usually starts with conventional 3-roll cane mills which are in
general use for this purpose.
In operating a mill after a diffuser, consideration must be given to the
difference in properties between mill bagasse and diffuser bagasse.
Firstly, it has a higher liquid content, averaging 650% on fiber. This
compares with about half this for bagasse entering the last mill of a
milling train at 200% imbibition on fiber. In fact, diffuser bagasse has
about the same relation between liquid and fiber as whole cane. Special
attention must, therefore, be given to drainage.
Secondly, the bagasse is at a higher temperature than normal for
mills. It is, therefore, more plastic and slicker, giving a lower coefficient
of friction with the rolls. This results in poor feedability. Greater
attention must thus be given to feeding devices and maintenance of mill
grooving.
Thirdly, diffuser bagasse is in a relatively fine state of subdivision.
Meeting these conditions effectively is a 3-roll mill fitted with a
heavy-duty pressure feeder. The settings should be more like that of a
first mill rather than that of a last mill because of the high liquid-fiber
ratio.
The characteristics of diffuser bagasse are particularly favorable to
dewatering in expeller type machines such as the screw press. In these,
high liquid content and high temperature favor feeding and reduced
friction in contrast to a roller mill. On the unfavorable side are the
relatively high power requirements and high maintenance costs.
PRESS JUICE TREATMENT
Juice from the dewatering equipment is returned to the diffuser in
order to recover some of the pol. Insoluble material in this juice may plug
the bed and cause diffuser flooding, so treatment is necessary.
Investigational work has shown that plugging is caused mainly by particles
40 microns or less in size. These fine particles form a layer of low
40
permeability by filling the interstices near the top of the bed. Juice
percolation through the bed slows and juice accumulates on the top of the
bed, a condition called flooding. Juice below the impervious layer drains
away, leaving a porous bed which results in poor juice contact. A
temporary flow can be induced by mechanically breaking up the layer, but
a short time later, the layer will form again farther down in the bed (Fig.
4-3).
It has been established that the particles that will cause plugging
settle rapidly in the low density juice, so that simple gravity settling
produces a relatively non-plugging juice. A conventional clarifier will
normally give a good juice without chemical treatment with a retention
time of about half that of lime defecation.
Fig. 4-3.
MINUTES
Press juice percolation through prepared cane bed.
41
The underflow from a press juice clarifier is difficult to handle on a
bottom feed rotary vacuum filter. The cake tends to form irregularly and
slough off on the feed side. Top feed filters perform satisfactorily.
Horizontal filters perform well and specially designed bottom feed rotary
machines with increased hydraulic capacity are also functional if bagacillo
is added.
OPERATION
At rated capacity, the Silver diffuser is designed for a target bagasse
pol of 1.0 at a draft of 100 (juice % cane). A cell rupture in prepared cane
of close to 94% is necessary. Based upon an average percolation rate of
2
2
250 liters/m (6.1 g a l . / f t ) at an optimum bed depth of 1.5 m (4.9 ft), the
diffuser speed would be 1.5 revolutions per hour. Under these conditions,
the fiber retention time in the diffuser is 37 minutes. The juice, however,
1
would have an average retention time of only half this (Fig. 4-4).
Permeability, Bed Depth and Diffuser Speed
When operated under design conditions (see flow diagram, Fig. 4-1),
juice from a distributor percolates through the moving bed and drains into
the next aft collection tank below. Deviation from standard conditions in
permeability, bed depth and diffuser speed would have the following
effects:
1.
If the bed permeability is lower, some juice is carried beyond the
next tank. This mixes the countercurrent flow, moving richer
juice toward the discharge end and reducing extraction.
Corrective action may be:
a.
b.
reduce bed depth
reduce diffuser speed
Both result in lower throughput.
2.
If the bed permeability is greater, some juice will recirculate and
countercurrent flow will be disturbed in the forward direction.
This, of course, is a good situation and can be corrected by
increasing bed depth or diffuser speed, both of which result in
increased throughput.
3.
If the bed depth is greater, the effect is similar to No. 1, low
permeability.
4.
If the bed depth is lower, the effect is similar to No. 2.
5.
If the diffuser speed is higher, the effect is similar to No. 1.
6.
If the diffuser speed is lower, the effect is similar to No. 2.
In order to compensate for variation in these conditions, the diffuser
has enough extra stages in its design to allow considerable intermixing of
flows. Changes of the order of 15%, therefore, have little effect on
performance. Changes of greater magnitude will show up in reduced
efficiency.
42
REF SOL.
FIBER
INSOL SOLIDS
WATER
REF SOL 2 . 5 4
FIBER
46.3
INSOL. SOL. 4 . 0
MOIST
47.2
POL
1.24
PURITY
48.4
D I F F U S E R FLOW BALANCE
PREPARED CANE T/hr. 100
DIFFUSER JUICE % CANE 1 0 0
EXTRACTION %
97
PRESS R E T U R N CAKE
2 . 0 T/hr
Fig.
4-4. Diffuser flow balance.
43
Cell Rupture
Cell rupture (Displaceability Index) below 94% gives higher bagasse pol
because of the slow rate of diffusion of sugar out of unopened cells. Rich
juice will, therefore, persist until the dewatering process, giving higher
pol in the returned juice, thus raising the pol level at the discharge end of
the diffuser. Good cell rupture is the single most important factor in
reaching high extraction. This is obtained by careful maintenance of the
shredder hammers and grids. As the surfaces of these wear and clearance
increases, the horsepower required to drive the machines increases. Thus,
horsepower is a good indicator of the type of preparation being obtained.
Periodic analysis for Displaceability Index is, of course, necessary.
Draft and Dilution
Draft is extracted juice plus dilution water as a percentage on cane, so
the 100 draft figure corresponds to a dilution of about 17% on absolute
juice for average cane. With a good countercurrent flow pattern and high
cell rupture, bagasse pol of 1.0 can be obtained with dilution rates below
17%. Under less favorable operating conditions, it is well to increase the
dilution and hence increase draft to the extent that the evaporator station
will handle the load.
The inventory of liquid in the diffuser must be maintained at such a
level that the bed is saturated. That means that the liquid level should be
the same as the bed level. If the liquid level is too high, above-surface
flow will result in unwanted mixing. If the level falls below the bed
surface, plug-type flow will no longer be possible and air filled channels
will result in poor liquid to fiber contact and lowered efficiency of
extraction. The bed should never be allowed to drain completely because
air pockets which form will remain, even if the bed is flooded again.
In order to maintain a saturated bed, the quantity of liquid must be of
1
the order of 1200% on fiber (Fig. 4-5). This amount of liquid is
maintained by sending to the diffuser all of the return from the
dewatering unit plus fresh water to make up the total draft. Return press
juice from the dewatering unit at 100 draft will be about 75%, requiring
about 25% fresh water. The flow of these streams must be steady and as
uniform as possible.
When the diffuser is stopped, juice flow is automatically diverted so
that it is pumped forward in complete countercurrent flow only at the
discharge end but recirculates on itself with only a small forward flow at
the feed end. This is to maintain a saturated bed. Shortly after a diffuser
stop, press juice will stop and, with automatic control, fresh water will
increase some four-fold to supply the running diffuser needs. The water
should be kept on for a short time, but on stops of more than a few
minutes, the water flow should be reduced to one-half rate after about 30
minutes.
Press Return
All juice from dewatering the bagasse is returned to the diffuser, just
as last expressed juice is returned to the tandem in milling. The return
must be relatively free of suspended solids and must have a minimum
Fig.
4-5. Juice flow as a percent of fiber.
44
45
temperature of 85° C. Quality with respect to bed-plugging material can
be tested periodically simply by observing the flow of a sample through a
100-mesh screen. The juice should pass through without any holdup on the
screen.
The press juice distributor should be inspected periodically to ensure
that there is no buildup of mud on the bottom.
Although treatment, other than settling, is normally not necessary, it
is advisable to add sufficient lime to bring the pH above 6 in order to
reduce corrosion of the equipment.
Settlings from the press juice clarifier need only rough filtration in
order to return the juice to the press juice heating tank. Since the pol is
low at this point, losses in cake are very small.
Temperature
Diffuser operation should be carried out under conditions in which
microorganisms will not grow. A safe temperature is 70° C. This is held
by keeping the press return and fresh water temperature at above 80° C
and heating the recycled juice at the front of the diffuser to the same
level.
Growth of microorganisms in the diffuser will not only destroy sugar,
but gum formation can seriously impair the permeability of the bed and
cause flooding.
It is well not to maintain too high a temperature as higher sugar losses
through inversion will occur. A t 70° C, the average inversion loss through
the diffuser, with unlimed juice (pH 5.2-5.5), is 0.14%. Also, lower
temperatures give less color development and less solubilization of fiber
components. This latter factor is responsible for the increase in nonsugar
dissolved solids in diffusion with a consequent decrease in juice purity.
Bagasse Dewatering
The dewatering unit must deliver a constant flow of return juice to the
diffuser and reduce the liquid content of the diffuser discharge from about
85% to less than 50%. The principal consideration is the maintenance of
good feedability. In the case of a mill, the grooving must be kept in
condition by frequent arcing.
DIFFUSER FLOODING
Failure of juice to percolate rapidly enough through the bed, resulting
in accumulation of liquid on top of the bed and, particularly, in front of
the discharge screws, is the principal problem encountered in operation.
As indicated previously, it is usually caused by plugging with finely divided
particles of soil.
The first indication of flooding is often seen in the irregular discharge
of bagasse from the screws. Liquid accumulating at the base of the screw
by poor draining of the bagasse prevents pickup by the screws. Bagasse
then, builds up until it reaches a high enough level that drainage has made
46
it dry enough to be picked up. Large quantities are then picked up until
the bagasse level falls and pick up ceases. In cases of serious flooding, the
bagasse may become so high that enough is picked up to overload the drive
motors on the screws.
The most common cause of flooding is return of press juice with
substantial quantities of fine suspended particles. The remedy is proper
control of overflow from the press return clarifier.
Usually, the heated press juice settles rapidly and presents no
problem. Juice from sour and deteriorated cane, however, may settle
poorly. Moderate chemical treatment such as lime or poly electrolytes has
not proven to be generally effective. Of course, extensive treatment such
as high liming, followed by phosphatation, which give a clear juice, will be
effective but expensive. The options available, therefore, are to slow
down the diffuser or to discard some of the press return replacing it with
clear water.
Another source of flooding is at the feed end, from recycling the juice
as previously indicated. This situation can be helped by stopping the
recycle and sending first pass juice to the boiling house. The juice will not
be as clear, but should not cause any difficulty in clarification.
A third cause may be just too much soil coming in with the cane, so
the whole bed is of low permeability. The obvious solution to this is
better cane cleaning. If this is not possible, the only thing to do is slow
down the cane rate.
An uncommon cause of flooding is growth of microorganisms in the
diffuser.
This can only occur when temperatures fall below 70° C,
permitting multiplication. Microorganism growth can give gum formation
which can almost gelatinize the cane bed and block it completely.
Devices which rake up the diffuser bed periodically have been used to
aid in percolation. Such action does enable juice to go through the bed but
poor extraction is the consequence.
CONTROL
Basic control parameters for a diffuser are:
1.
2.
3.
4.
Cane feed rate
Displaceability index
Diffuser speed - bed depth
Dilution water rate
No sensing device has been developed for the governing element which
determines how these parameters, with the exception of displaceability
index, will be set, namely the bed permeability. This condition is judged
solely by observation and operating set points plugged in accordingly.
Cane feed is weighed on a belt scale, the feed to which is controlled by
the chest pressure on the turbine driving the Buster. Feed to the Buster is
controlled by the Buster feed roll. Cane in the chute to the Buster is
47
sensed by a level arm riding on the cane which actuates a switch to the
carrier drive motor.
Speed of the diffuser is set by the cane feed rate so as to maintain a
minimum bed depth of 1 m and a maximum of 1.5 m.
Displaceability index can only be determined by periodic sampling and
laboratory analysis. However, a good idea of the degree of preparation
can be had by visual observation. Also when the hammers become worn
the horsepower of the drive increases. This means poor preparation and
thus a low displaceability measurement.
Rate of dilution water under standard conditions is that necessary for
100 draft. This means that the total of juice returned from dewatering
bagasse (press juice) plus dilution water is equal to the cane rate. Since
the return juice normally is around 75% of the cane, the water added is
about 25% of the cane. It is often the case that the evaporator capacity
is limiting. Under this condition, the quantity of water is that which the
evaporator will handle, then the draft may be less than 100. Likewise,
with ample evaporator capacity, additional water can be used to lower the
bagasse pol. Water temperature should be 80° C.
REFERENCES
1
Payne, J. H., Cane Diffusion - The Displacement Process in Principle
and Practice, Proc. Intern. Soc. Sugar Cane Tech., 1968, pp. 103-121.
49
Chapter 5
BAGASSE MOISTURE
Although the moisture content of final bagasse is governed primarily
by the performance of the last mill, the composition of the material
entering this mill has a significant effect. This is determined by the
quality of the entering cane and the extraction in the preceding mills.
Assume cane entering the tandem at 15% fiber and 15% dissolved
solids, the entering moisture is 70%. Juice extracted by dry crushing in a
2-roll crusher and a 3-roll mill should be 77%. Then, the residual juice in
first mill bagasse would be:
(100 - 77) χ 0.85 χ 100
_
(100 - 77) χ 0.85+ 15
" ™ ™
Λ
and the moisture would be:
70 χ 0.23 χ 100
15 + (15 χ 0.23) + (70 χ 0.23)
,
i 4 Rf O
4
b
,
b
*
leaving fiber of
43.4%
moisture plus fiber
90.0%
and dissolved solids of
10.0%
In the following mills of the tandem, the application of the
countercurrent maceration system results in the replacement of most of
the juice with water. If approximately the same relationship of solid to
liquid (fiber to juice) is maintained in subsequent mills as in the first, then
bagasse leaving the last mill with a juice of 3.0 dissolved solids would have:
dissolved solids = 56.5 χ 0.03 = 1.7%
and moisture = 56.6 - 1.7 = 54.9%
Thus, the moisture content would increase as the dissolved solids decrease.
In order to reduce the moisture, it is necessary to increase the fiber
content progressively down the tandem. The mill settings must,
therefore, be reduced, increasing the fiber, the last mill setting
determining the moisture content of the final bagasse.
With no mill slippage in a properly set mill, the water content of
bagasse leaving a mill is independent of the water content entering. This
is true because the load on the mill is determined by the fiber and not
fiber plus liquid. However, with slippage, the water content of the
expelled bagasse increases since the fiber entering the mill is less and
space is available for more water to pass.
Maintenance of feedability is, therefore, of prime importance on the
50
last mill. This requires constant attention to building up of roll grooving
and effective operation of the feeding aid.
Keeping in mind that the higher the dissolved solids in the residual
juices (poorer extraction), the lower is the bagasse moisture at the same
fiber content; the fiber content of bagasse is a more useful measure of
last mill performance than moisture.
Effect on Extraction
Other conditions being the same, a reduction in the moisture in
bagasse results in higher extraction. The actual increase is dependent
upon the fiber, moisture, pol, and refractometer solids involved. By using
average figures for these a general relationship as shown in Figure 5-1 is
obtained. From this it will be seen that a decrease in bagasse moisture of
1% results in an increase in extraction of approximately 0.1%.
53
52
51
50
49
48
47
46
45
44
43
42
MOISTURE % BAGASSE
Fig. 5-1. Effect of bagasse moisture on extraction.
41
40
51
The calculations assume little change in the pol of the juice extracted
and are illustrated in the following example:
Assume pol in cane
Bagasse fiber
pol
refractometer solids
moisture
13.5
48.0
1.44
2.32
49.7
Then extraction =
97.0
Now increase fiber by one unit
Bagasse fiber
pol
refractometer solids
moisture
49.0
1.41
2.28
48.7
Then extraction =
97.1
If the bagasse fiber is coarse, the increase in pol extraction may not be
as great because the pol of the juice within the fiber may be higher than
that of the external juice. The applied imbibition water will not have
reached equilibrium with the internal juice. This effect may be
counter-balanced to some extent if the lower moisture level has been
obtained by higher pressure on the mill or a greater preparation effect in
the mill, both of which would tend to give closer to equilibrium conditions.
Effect on Fuel Value
The higher fuel value of bagasse at lower moisture content can be
reasonably calculated but the practical value obtained varies considerably
with the boiler and how it is operated. For rough estimations, however, a
change of 1% in moisture changes the fuel value also by 1%. An
illustrative example follows:
At 50% Moisture
Analysis
Per 100 Tons Bagasse
Fiber
Moisture
Soluble solids
%
Tons
48
50
2
48
50
2
Total
100
Approximate Net Calorific Value
or
1853.7
1853.7
3336 χ
7760 χ
kg-cal/kg (3336 Btu/lb) (7760 kJ/kg)
χ 1000 χ 100 = 185,370,000 kg-cal/100 metric tons
2000 χ 100 = 667,200,000 Btu/100 short tons
1000 χ 100 = 776,000,000 kJ/100 metric tons
52
At 49% Moisture
Analysis
Per 100 Tons 50% Moisture Bagasse
Fiber
Moisture
Soluble solids
%
Tons
49
49
2
48
48
__2
Total
98
The weight of bagasse is now 98 tons.
Approximate Net Calorific Value
1903.1
or 1903.1
3425 χ
7967 χ
kg-cal/kg (3425 Btu/lb) (7967 kJ/kg)
χ 1000 χ 98 = 186,507,000 kg-cal/100 metric tons
2000 χ 98 = 671,300,000 Btu/100 short tons
1000 χ 98 = 780,766,000 kJ/100 metric tons
The increase in Net Calorific Value is therefore
186,507,000 - 185,370,000 = 1,137,000 kg-cal/100 metric tons
671,300,000 - 667,200,000 = 4,100,000 Btu/100 short tons
780,766,000 - 776,000,000 = 4,766,000 kJ/100 metric tons
or
1,137,000 χ 100
= 0.61%
185,370,000
The actual value obtained in a boiler would be higher, approaching 1%.
In calculating the fuel value of bagasse, it is customary to include the
energy value of the sugar as well as the fiber. It should be kept in mind
that this is valid only if the bagasse goes directly from the mill to the
boiler. In storage, the sugar content is rapidly destroyed so that after
several hours it can be considered to be zero.
53
Chapter 6
THE IMPACT OF EXTRANEOUS MATTER
ON MILLING AND DIFFUSION
Extraneous matter is the non-cane material in cane delivered to the
factory. This is often referred to as trash, although the meaning of this
term varies in cane sugar producing areas. In its original use, it referred
to leaf material accompanying the cane stalk as harvested. This concept
still obtains in many areas. A t present, the broadest definition is that
officially used by the Hawaiian Sugar Technologists, which includes all
extraneous material delivered with the cane stalk, from root to growing
point, and covers not only cane leaves, but also the growing tip and roots,
weeds, soil, rocks, casual matter such as dump refuse, structural material,
machinery parts, livestock and even superficial water - an important item
in washed cane.
Purely quantitative figures on total extraneous matter do not provide
adequate information for predicting the effect on operations. It is
necessary to know the physical characteristics of the foreign materials.
Five main categories, with subclassifications, cover most cases and give
useful characterization.
1.
Fibrous material
Dry leaves
Tops
Ground trash (partially decayed material)
Roots
Dead cane
Weeds
2.
Soil
Clay
Loam
Sand
3.
Rocks
Gravel
Stones
4.
Metal (tramp iron)
5.
Water
A consideration of the general effect of these on milling and diffusion
follows.
54
MILLING
In engineering calculations, the capacity of a milling tandem is
customarily considered to be a function of the fiber throughput per unit
time. Actually, the quality of the fiber also has a bearing but is difficult
to quantify. Therefore, the calculated quantity of cane crushed in a mill
is judged by the fiber content of the cane plus the fiber content of the
extraneous material entering the tandem with the cane. In a given area,
the fiber content of sound mature cane stalk varies within narrow limits
(Hawaiian varieties 12-14%).
The amount of fiber entering with the cane as extraneous matter is
subject to wide variation, however, dependent upon the effect of burning,
field and harvesting conditions, as noted in Chapter 2.
The amount of fibrous trash entering with the cane ranges from 4% for
well-burned cane on an irrigated plantation to 25% for unburned cane on
an unirrigated plantation. The fiber content of the trash also varies from
a low level of 10% for ground trash harvested under wet conditions,
through 25% for green unburned tops to 80% for dry leaves.
Using the estimated figure of 40% fibrous trash removal in the cane
cleaner, the range of fibrous trash entering the mill is from 2.5 to 15%
based on the entering moisture content. After the cleaner, however, the
material is completely saturated with water regardless of the entering
moisture. Fiber levels then are:
Green tops
Leaves
Ground trash
25%
35%
10%
A rule-of-thumb value for the fiber content of fibrous trash entering
the mill from the cleaner would be twice the fiber content of the cane, or
24 to 28%. Thus, in milling calculations, one ton of fibrous trash is
approximately equal to two tons of stalk cane.
If the fibrous trash is clean, the major effect of it would be to increase
the fiber milled. Addition of 10% trash to clean cane would necessitate a
20% increase in capacity of the mill to handle the same amount of cane.
Should the mill have sufficient capacity to handle the higher fiber
rate, the setting can be adjusted so that the pol and moisture in bagasse
can remain unchanged. Only the quantity of bagasse will increase, which
means a greater loss in pol and lower extraction. Imbibition water would
have to be increased proportionate to the fiber.
The approximate magnitude of the loss can be illustrated
example.
Assume
Cane
100 tons
12.5% pol
12.5% fiber
in an
55
Bagasse
2 5 tons
2.5% pol
50.0% fiber
.". Pol lost in bagasse =
25 χ 0.025 = 0.625 tons
and Extraction =
12.5 - 0 625 x 100
1
ù
=
9 5
0 %
. 0
Now add 10% fibrous trash of 25% fiber
Cane
110 tons
12.5x^100
Bagasse
=
1
.
I
3
6
%
12.5 + 2 . 5 x 100
=
1
3
p
>
ol
6 %
4
f i b re
30 tons
2.5% pol
50.0% fiber
. ' . Pol lost in bagasse =
30 χ 0.025 = 0.75 tons
and Extraction =
12.5 - 0.75 x 100
12.5
94%
Therefore the combined effect of adding 10% fibrous trash would be to
decrease the mill capacity by 20% and reduce extraction by 1%.
A compensating factor is that the quantity of bagasse would be
increased by 20%. The value of this as fuel for generating power would
more than compensate for the loss in extraction.
There are other effects associated with trash which may be of greater
and even governing importance. Even with normal cleaning, fibrous trash,
particularly ground trash, will carry into the mill soil, sand and rocks.
These materials cause heavy wear on the equipment - knives, hammers,
mill rolls, pipes and pumps. The result is a decrease in cane preparation
and a lowering of the juice expression by the mills. Polishing of rolls will
also decrease feedability and, thus, lower the capacity. The net effect
can thus be a decrease in capacity of the mill substantially greater than
that caused by the fiber alone. Extraction will also be lower than that
caused by the fiber alone.
Another factor is that soil carried into the mill decreases the
permeability of the bagasse blanket holding the juice and lowers juice
drainage, giving decreased extraction.
56
Trash is the cause of excessive water entering the mill from the
cleaning plant since clean cane stalks can carry no more than about 2%
superficial water. Green leaves, likewise, carry little water due to their
waxy surface. Dry leaves and ground trash, however, can carry a large
amount of extraneous water. This superficial water adds to the dilution of
the juice without aiding extraction.
In day-to-day operation of a mill, the effects discussed above will only
be observed but cannot be measured. Experience alone will enable the
operating staff to make the necessary judgment in appraising the daily
results.
DIFFUSION
The discussion of the effects of extraneous matter on milling applies
also for diffusion. The different technology presents a somewhat
different picture, however.
The capacity of a diffuser is determined primarily by the permeability
of the bed of prepared cane, that is, the rate at which liquid can flow by
gravity through the bed. The physical character of the fiber and the
effect of the extraneous matter on this, therefore, are governing. The
capacity then is the volume of juice that can pass through a quantity of
cane per unit time. The size of the diffusion vessel, finally, is determined
by the volume the bed occupies at an optimum bed depth.
In diffuser design, a target extraction, say 98%, is set; then a
practicable cell rupture value (Displaceability Index), say 94%, selected
and the number of stages necessary calculated. A practicable bed depth
has been found by experience to be about 1.5 m (5 f t ) . Bed permeability
then determines the length of the bed at a given speed. To compensate
for variability in permeability, more stages than calculated are normally
designed. Some flexibility in operation is achieved by variation of bed
depth and speed. Capacity of the diffuser then becomes a function of the
third dimension, the width.
Preparation of the cane was noted as the most important factor in
diffusion efficiency. Maximum cell rupture, to free the juice, is necessary
at the same time giving a fiber which is highly permeable. This was shown
to be best obtained by a shredding machine which produces relatively long
shreds resulting in minimum compaction when formed into a bed by
gravity. Well-prepared cane will have a permeability averaging 250
liters/m^ (6.1 gal./ft2). - Fibrous trash, leaves and tops, generally make
shredding of cane more difficult and greatly increases power required.
Extraneous matter can have a controlling effect on diffuser
performance mainly by the influence on bed permeability. Fibrous trash
will increase the volume of the bed and so the capacity of the diffuser will
be less in proportion to the volume occupied by the trash fiber. The
effect is variable, depending upon the physical character of the trash. In
general, shredded trash fiber occupies more volume per unit weight than
shredded cane so that the capacity influence is greater than indicated by
the weight of the trash.
57
The effect of clean fibrous trash on permeability is not great.
Shredded dry leaves and green leaves can improve permeability. This is
particularly true of green leaves, being waxy, improving liquid flow.
Ground trash, being spongy, can have an opposite effect. However, unless
the material is extremely fine (less than 300 mesh), fibrous trash does not
have a major influence.
Soil particles are the most serious suppressors of permeability. Clay
type soils can almost completely seal a fiber bed and essentially stop
diffusion if present in sufficient quantity. This leads to flooding - the
most serious diffuser difficulty of all. With material finer than 300 mesh,
the reduction in the permeability is proportional to the amount of the fine
material up to a point - after which the amount is immaterial. This is
because when an almost impermeable layer is formed, little liquid will
pass through and the remainder will flow away on the surface of the bed.
Other materials which can effect bed permeability are precipitated
substances such as clarifier settlings. These, if returned to the diffuser,
must be distributed through the bed and not placed on top. Sand, grit and
rocks, of course, are without effect.
In conclusion, as to the role of extraneous matter on diffusion, the
fibrous trash has the effect of reducing capacity in proportion to its
volume. Any other extraneous matter, soil in particular, of a fine particle
size that decreases the bed permeability, can have a governing effect on
the capacity of a diffuser. The impact on the preparatory and dewatering
equipment is much the same as on milling. Because bagasse pol is lower in
diffusion the effect of trash on pol loss is substantially less than in milling.
59
Chapter 7
A COMPARISON OF DIFFUSION AND MILLING
WITH RESPECT TO RECOVERY AND LOSSES
A question often raised, and obviously not subject to definitive answer,
is how do diffusion and milling compare from the standpoint of recovery
and losses. As with all equipment, it depends upon the capability of the
installations and how they are used. There are, however, some basic
principles to consider in looking at the technologies involved, and hence
arriving at reasonable relative conclusions.
Since the conventional extraction figure incorporates cane quality as
well as extraction efficiency, it is more useful to look at bagasse pol as a
measure of processing results. Basic diffuser design sets a target of 1.0%
pol in bagasse. With good quality cane, this would correspond to
extraction at the 98% level.
Although in common practice mills are not designed to a bagasse pol
target, the range for a good operation is 1.5 to 2.0 pol. Thus design
consideration are based on losses in bagasse by diffusion 33% to 50% less
than those for a mill. The extraction level for a mill is therefore 96%
compared to 98% for diffusion.
Diffusion takes place in closed equipment
growth range for microorganisms. Loss by
essentially nil. This contrasts sharply with
perature system optimum for microorganism
1% have been reported.
at a temperature above the
their activity is, therefore,
milling - an open, low temactivity where losses above
Chemical inversion of sucrose, a product of temperature and pH, is
negligible in a diffuser if a near neutral pH is maintained, since the
average juice residence time is less than 15 minutes. With unlimed juice
(pH 5.2), the calculated loss by inversion is 0.14%. With a mill, it would
be less than 0.05%.
It is true that the purity of diffusion juice is lower than the purity of
absolute juice, as determined by direct disintegrator analysis of cane.
However, it has been established that the purity change is not caused by a
loss of sucrose but by an increase in dissolved solids (Brix). This is
attributed to a solubilization of some high molecular weight components
of the cane in the diffuser. No losses are involved therefore.
It may be concluded from the above considerations that recovery by
diffusion should be of the order of 2% greater than that by milling, in
properly designed equipment operated at rated capacity.
A good practical comparison of milling and diffusion is given by the
results in Hawaii obtained by Hawaiian Commercial and Sugar Company in
its two factories, Puunene and Paia. The factories are adjacent to one
another and operate under as similar conditions as it is possible to find.
60
The Puunene factory had two milling tandems, each with four 198 cm
(78 in.) mills preceded by a shredder and a 2-roll crusher. The Paia
factory had a Silver 840 diffuser (2134 cm, 840 in. diameter) preceded by
a Buster and Fiberizer, and followed by a 214 cm (84 in.) 5-roll pressure
feeder dewatering mill.
The results for the 1975 operating season were as follows:
Extraction
Bagasse pol
Bagasse moisture
Mixed juice purity
Boiling house recovery
Total recovery
Puunene
Paia
95.1
2.6
45.5
85.6
89.2
84.8
97.3
1.4
45.7
86.4
89.3
86.7
61
Chapter 8
CLARIFICATION
The main objective at the clarification station is to increase the pH of
the juice to a level where losses of sucrose by inversion are kept to a
minimum in the subsequent sugar recovery process. Important, but
secondary objectives, are the removal of insoluble material and some
undesirable dissolved substances. It is fortuitous that adjustment of pH to
the optimum level with the cheapest alkalizing agent, lime, gives
satisfactory removal of much of the undesirable material in the juice and
provides a suitable base for sugar recovery. Magnesium oxide behaves
similarly to lime and is often used to reduce evaporator scaling.
Target for pH adjustment of the juice is that which will give a syrup
pH of 6.5. This is about the optimum for carrying out the crystallization
steps which follow, by giving easy boiling massecuites, minimum
development of undesirable compounds and color, small decomposition of
reducing sugars and little loss of sucrose by inversion. A t higher pH
levels, there is greater development of viscosity and color and substantial
losses of reducing sugars, particularly fructose. A t lower pH levels
sucrose inversion increases rapidly. Processing a syrup at 6.5 pH will
usually give a final molasses pH of around 5.8, so the crystallization is
conducted in the range of 6.5 to 5.8. The mixed juice will have to be
raised to a pH of 7.5 in order to obtain a syrup of 6.5 because of the drop
which occurs in the juice heaters, clarifiers and evaporators. This
increase in acidity is caused by the relatively slow reaction with lime and
particularly magnesium oxide in the cold, by formation of organic acids
and by loss of ammonia from the decomposition of amino acids.
The exact pH of juice liming required varies with the composition of
the juice, so routine adjustment of set point is essential. Usually, with
good quality cane, good clarification also occurs under this type of
control. That is, there is good flocculation of suspended matter, rapid
settling and overflow of limpid juice. With poor quality or deteriorated
cane it is often impossible to get a clear juice and rapid settling.
Milky-appearing juice is a sign of sour cane. It is caused by dextrans
which, by protective colloid action, prevent good flocculation. In such
cases, higher liming is sometimes helpful even though the effects on sugar
crystallization are less favorable, so a compromise is necessary. In most
cases, however, there is little that can be done to improve the situation.
POLYELECTROLYTES
Polyelectrolytes are the one class of additives, besides the alkalizing
agent, that is so generally effective that its use in clarification is
routine. In very small quantities, one to two parts per million of juice,
62
polyelectrolytes improve flocculation, give more rapid
smaller volume of settlings. They have no effect on pH.
settling
and
Standard polyelectrolytes are partially hydrolyzed Polyacrylamides.
The pioneer agents, used under the trade name of Separans, remain as
standard. Preparation of the solution and point of application are
important. High purity water, such as condensate, should be used to
prepare the solution. Water with high dissolved solids or suspended solids
will give a solution with inferior floe forming characteristics.
The best point of application is after the juice is heated and a good
place is in the flash tank of the clarifier. A metering pump is necessary
to control the addition.
The useful quantity of polyelectrolyte varies with the quality of the
juice and the type of soil present. Usually, one to two parts per million of
juice is sufficient. U. S. government regulations limit the quantity to not
more than 5 parts per million.
MAGNESIUM OXIDE
Magnesium oxide (Magox) is an effective alkalizing agent for replacing
lime becuse it leads to less scaling in the evaporators. Evaporator scale is
composed mostly of calcium sulfate and silica. Since magnesium sulfate
is soluble, less calcium sulfate is deposited on the tubes than when lime is
used.
Magnesium oxide has 40% more basicity per unit weight than quick
lime, but even so is more expensive to use. When scaling is a problem, it
can be used to replace enough of the lime to control the scaling. The
amount must be determined by trial. Often, a 50-50 ratio is adequate.
As the solubility of magnesium oxide is low, its rate of reaction with
juice components is slow, making addition control by pH unsatisfactory
unless a retention time of some 15 minutes is allowed. Although there is
lag time in the heaters, better control is obtained from heated juice
entering the clarifier.
The quality of the magnesium oxide is extremely important as,
depending upon conditions of calcination, the product can be highly
absorptive
(low temperature)
or practically
nonabsorptive (high
temperature). For clarification use, the specifications are loss on ignition
2.5% and iodine number 20.
PHOSPHATATION
Clarification of juices deficient in natural phosphate is sometimes
helped by increasing the phosphate content. In general, juices containing
less than 0.03% phosphate are considered deficient. Adding phosphate to
this level gives more calcium phosphate floe and often better clarity.
Care must be exercised, however, because increased volume of settlings
results and, usually, slower rate of settling.
63
Juices with excess natural phosphate (around 0.09%) are slow settling
and give large volumes of settlings. Clarification of these is sometimes
helped by reducing the liming pH.
The best source of phosphate is phosphoric acid, a liquid which is
expensive and presents a problem in handling. Normal practice is to use
the cheapest and most available form which is fertilizer grade ammonium
phosphate.
SULFIT ATION
Sulfur dioxide is widely used, both in cane and beet sugar processing,
to reduce sugar color. It also has a secondary effect in improving the
boiling characteristics of massecuites.
The action of sulfur dioxide on color is complex, involving converting
colored compounds to colorless, preventing color formation by oxidation
and inhibiting color development from reducing sugars and amino acids by
the browning reaction.
Addition of the sulfur dioxide gas may be made before or after liming,
with little apparent difference in effectiveness. The simplest application
is to control the quantity entering the mixed juice relative to juice flow,
then carry out the customary liming by pH control. In order to avoid the
corrosiveness of sulfured mixed juice, partial preliminary liming may be
used.
To simply improve the quality of raw sugar, it is not necessary to have
substantial absorption or retention equipment. Sulfur dioxide from a
standard sulfur burner can be introduced proportional to flow into
prelimed mixed juice by means of a venturi unit. A certain percentage of
juice recycling will give uniformity. Liquid sulfur dioxide may be
introduced directly into the mixed juice line, controlled by means of a
flow meter.
The quantity of sulfur dioxide used in improving the color of raw sugar
is in the range of 100 to 500 ppm. This compares with around 1000 ppm
used in the manufacture of a direct consumption plantation white sugar.
JUICE HEATERS
The objectives of juice heating are elimination of microorganisms by
sterilization, completion of chemical reactions with the alkalizing agent,
flocculation and removal of gases. Effective gas elimination is obtained
by flashing as the juice enters the clarifier. The juice temperature must,
therefore, be raised above the boiling point at atmospheric pressure
which, at sea level, means a minimum of 103°C (217°F). If flashing does
not occur, gas bubbles adhering to the floes decrease the rate of settling.
Good steam economy is obtained by heating with vapors from the
evaporator. Usually this is done in two steps with primary heating by
vapors bled from the second or third effect and secondary heating with
vapors from the first effect. The efficiency of heating is primarily a
function of the heat transfer between the vapor on the outside of the
64
tubes and the liquid within. The optimum juice velocity is 2 m (7 f t ) per
second which gives good heat transfer without too high pressure drop. The
governing factor, however, is scaling of the tubes so periodic cleaning is
required. Heater scale is relatively soft and can often be removed by
cracking with steam followed by flushing with water. If this is not
effective, flushing with caustic soda solution will usually remove the scale.
Good heat transfer also requires removal of noncondensible gases and
condensate discharge from the heaters.
ROTARY VACUUM FILTERS
Settlings from the clarifier, which contain 5 to 10% insoluble solids,
are handled on a rotary vacuum filter to remove most of the insoluble
material and wash the juice from it. The juice from the settlings, along
with the washings, are returned to the incoming juice.
A diagram of a vacuum filter station is shown in Figure 8-1.
The filter is a rotating drum, the lower part of which is immersed in a
tank of settlings. The drum is made of independent filter sections covered
with a screen, usually of stainless steel with 0.6 mm (0.023 in.)
perforations. The filter sections are connected by piping to a manifold at
the end of the drum. The manifold opens to a rotary port valve which
controls the connection to a vacuum system. See Operating Action
Diagram, Figure 8-2.
The filter sections immersed in the tank containing clarifier settlings
mixed with bagasse fines are opened to a low vacuum of 18 cm (7 in.) of
mercury called pickup. In this position, liquid flows through the screen
and a cake is formed on the screen.
After the formation of the cake, the bagasse fibers begin to retain the
insoluble particles and the liquid passing through becomes relatively clear,
as distinct from the first influx which is unfiltered. As the section
continues to rotate through the settlings, the cake becomes thicker until
it finally emerges from the settlings. Here, the valving connects with a
high vacuum system of up to 50 cm (20 in.) of mercury and water is
applied to the surface by sprays. The water passes through the cake,
washing out the juice. A t the top of the drum, water is added by means of
drip pipes which keeps the cake wet and the vacuum sealed until the
vacuum is released and the cake scraped from the drum.
The juice and washings are pumped to mixed juice and the cake is
discharged to waste.
Bagasse fines are used as a filter medium for retaining the suspended
solids. These are obtained by screening bagasse and are mixed with the
settlings before going to the filter tank.
Bagasse Fines
Fines for the filter are commonly obtained by means of a screen fixed
in a bagasse chute. A suitable screen is flanged-lip type with 0.3 χ 0.5 χ
2.0 cm (1/8 χ 3/16 χ 3/4 in.) slot dimensions at head, tail and length.
Fig.
8-1 Vacuum filter station.
65
66
A.
Β.
C.
D.
Fig.
PICK UP
CLOSE
WASH
DISCHARGE
8-2. Vacuum filter operation action diagram.
67
2
2
Minimum area required is 500 c m (0,5 f t ) per ton prepared cane per hour
capacity.
The fines are pneumatically conveyed to a cyclone above the settlings
mixer. A t the collection point, the stream of the fines is adjusted to fall
past the intake in a manner so that sand drops out of the stream by air
separation.
Good quality screening should provide fines, 90% of which will pass
through a 14-mesh screen. The material should contain fibers as well as
pith in order to have good permeability.
Mixing Fines with Settlings
Fines are mixed with the settlings in a ribbon screw mixer. The
fluidity of the mix must be maintained in order to prevent piling up of the
bagasse on the surface. Since fluidity depends upon the consistency of the
settlings, as well as the quantity of added fines, the ratio of fiber (fines)
to insoluble solids that can be maintained is variable. A minimum ratio or
0.35 is considered standard and up to 0.5 is desirable. To maintain a 0.5
ratio, a quantity of bagasse fines equal to that of the insoluble solids must
be added because the fines are 50% moisture. Thus, 100 tons of mixed
juice at 1% insoluble solids would need 1 ton of fines.
Pickup Vacuum
At the filter pickup point, the vacuum applied should be just enough to
hold the cake, but not enough to compress the cake that will reduce
permeability; 18 cm (7 in.) vacuum is optimum. If a high vacuum is
applied here, the cake will be compressed and the subsequent wash water
will not penetrate in sufficient quantity to displace the juice.
Wash Vacuum
On the wash cycle, the vacuum must be high enough to provide juice
displacement flow through the cake. A minimum vacuum of 38 cm (15 in.)
is standard and 50 cm (20 in.) is the upper limit so that excessive flashing
will not occur.
Washing
The cake must be uniformly covered with water during the wash
cycle. It should not be allowed to dry out. Otherwise, the cake will
crack, vacuum will be lost and cake penetration cease. The cake should
always have a shiny, wet surface. Initial washing is with sprays set to give
enough water to keep the cake wet without running down and eroding the
surface. For these sprays to operate properly, clean water must be
supplied at constant pressure. A pressure control valve is used on this line
and a screen is essential. The water should be at a minimum temperature
of 70° C. With a cake of good physical characteristics the quantity of
wash water necessary can be as low as 0.5% on mixed juice and should not
exceed 1.0%.
Speed
Standard filter speed is 10 revolutions per hour. The speed may be
increased under some conditions and, in particular, with high ratios of
68
fiber to insoluble solids. In general, it is desirable to operate at the
slowest speed compatible with a given composition of the settlings.
Cake Pol
Standard target for cake pol is 1.0%.
conditions, the pol should be lower than this.
Under good
clean
cane
Cake Quantity
A rule-of-thumb indicator of the quantity of cake is:
Filter cake = 4 χ insoluble solids in mixed juice + 0.01 gross
mixed juice
CLARIFIE RS
A clarifier
alkalizing step
sterile product,
give a syrup of
of:
1.
2.
3.
4.
5.
6.
should provide the means of withdrawing juice from the
in a condition suitable for sugar recovery. This means a
relatively free from insoluble matter and at a pH level to
6.5. The equipment must, therefore, supply the functions
Gas removal
Settling
Scum removal
Clear juice drawof f
Settlings thickening
Settlings removal
Present practice makes use of equipment in which the treated juice
enters continuously with simultaneous withdrawal of clear juice, settlings
and scums. The best design is that giving the minimum flow velocity at
the entry and drawoff points, minimizing interfering currents. Units with
multiple feed and drawoff points are less amenable to control.
Flashing
Clarifiers are equipped with a flash tank where the juice first enters
and boils at atmospheric pressure, eliminating gases. The effectiveness is
more a function of the area rather than the volume. Commonly, flash
tanks have too small an area. The La Musse * formula is useful:
Area for flash in m2 = 4.186 W ( T l - Tg)
L
Where
W
= Weight of juice in metric tons/hr
T\
= Temperature entering, ° C
T2
= Temperature at flash, ° C
L
= Latent heat of steam at T 2 , kcal/kg
For a juice rate of 221 tons/hr and temperatures of 103 and 100°C,
2
respectively, the formula calls for 5.15 m area.
69
Settling Space
The capacity of the clarifier is determined by the retention time
necessary to allow the settling of a filterable mud. The most efficient
design in this regard is the single mud compartment, multiple launder
drawoff, of which the SRI (Australia) is a version. A retention time of 30
minutes or less is possible in these types. This compares with the
multitray and multimud compartment units with retention time in the
range of 2 to 2-1/2 hours. A disadvantage of short retention time
clarifiers is that the juice flow must be relatively constant, for there is
little surge capacity.
The area involved in settling is a major factor in efficiency. The
greater the area per unit volume, the more rapid the settling and the
smaller the volume of settlings. Multicompartment clarifiers suffer more
from uneven flow distribution and disturbing currents, even though total
area may be adequate.
Operation of Multicompartment Clarifiers
Control of a multicompartment clarifier remains a manual operation.
Drawoff of clarified juice is controlled by adjusting the drawoff valves to
maintain the desired clarity, reducing the flow from those giving cloudy
juice. This can be a time-consuming job since reducing the flow from one
increases the flow from others in unequal amounts.
Settlings drawoff is usually set to maintain a minimum consistency of
5% insoluble solids so that good operation of the filters is possible. With
clarifiers having one mud compartment, this is relatively easy. For those
with two mud compartments, it is difficult and for those with four, it is
more difficult. Usually, more mud enters the first and last mud
compartments than those in-between, so adjustments must be made
accordingly. However, changing the amount of drawoff from one changes
all the others and all the clarified juice drawoffs as well.
As the quality of the incoming juice changes, so must the operation of
the clarifier. The common solution to this problem is to provide excess
clarifier capacity. This is not only capital expensive, but also increases
losses because of longer retention time.
The clarifier should be kept full at all times and not be used as a surge
tank by allowing the level to drop.
Holdover Juice
Normal sucrose losses in clarification, exclusive of filtration, are of
the order of 0.2%. The figure includes sucrose inversion, destruction and
handling losses. Losses where juice is held in the clarifier for longer
periods, such as weekend shutdowns, is subject to continued losses
principally by inversion. These depend upon the temperature and the pH
of the juice.
In order to keep losses to a minimum, the temperature must be kept
above 70°C to prevent the growth of microorganisms. The pH tends to
70
drop with storage, so addition of soda ash is useful to prevent a fall below
6.0. Normally, juice should not be held over 24 hours because of the
difficulty of maintaining the temperature. The growth of organisms
cannot be tolerated, as not only loss of sucrose occurs but subsequent
sugar boiling operations are affected.
Clarified Juice Screening
Clarified juice usually contains a small amount of fine bagasse. This
should be removed by flowing through a hillside screen of 80 mesh.
REFERENCES
1
LaMusse, J. P., South African Sugar J. 61, (1977), 103.
71
Chapter 9
EVAPORATION
The evaporation station performs the first step in the process of
recovering sugar from juice - the evaporative removal of water. Standard
practice is to concentrate the clarified juice to about 65 refractometer
solids which requires removal of approximately 75% of the water. Steam
economy dictates the application of the multiple effect principle. An
appropriate installation makes use of a quadruple or quintuple effect
arrangement with sufficient capacity to evaporate the water and is so
designed to provide vapor for juice heating and vacuum pan operation.
The station also furnishes boiler feed water from condensate.
MULTIPLE EFFECTS
In multiple effect evaporation the vapor from a vessel of boiling juice
is used as the source of heat for a subsequent vessel. This can be done by
reducing the pressure in the second vessel so that the boiling point is
lower. In a series arrangement, or multiple effect, the Rillieux Principle
states that one unit of steam will evaporate as many units of water as
there are vessels, or effects. Thus, in a generally used four-unit series, or
quadruple effect, one unit of steam would evaporate four units of water.
The quantity is not absolutely correct but is close enough for routine
estimations. Factors which cause deviations are:
1. Heat necessary to raise the temperature of the juice to the boiling
point in the first effect.
2.
Heat losses by radiation and removal of incondensible gases.
3.
Increase in latent heat of the vapor as the temperature decreases.
4.
Decrease in specific heat of the juice as it is concentrated.
5.
Flash evaporation of the juice as it enters a lower pressure effect.
6.
Condensate flash.
The first three of these tend to decrease the total evaporation while
the last three tend to increase the evaporation. The overall effect found
in practice is that the total evaporation is somewhat less than the
principle states.
Calculations based upon estimations of the values of the above six
factors show that the evaporation rates of the first three effects of a
quintuple are less than shown by the Rillieux Principle whereas those of
the last two, and in particular the last effect, are larger. However,
because of scaling this is not found in practice.
In order to have effective heat transfer, there must be an adequate
temperature drop across each heating surface in order to transfer heat
72
through the tube to the juice, so the number of effects practicable is
limited. The temperature in the calandria of the first effect is
determined by the pressure of the exhaust steam available. Although it is
advantageous to use higher pressure steam, general practice in cane sugar
2
2
operations is to utilize exhaust steam at not over 1.05 k g / c m (15 l b / f t )
(103 kPa) to enable better power production from the turbines. If the
steam is saturated, the temperature would be 121° C. Actually, because
of loss in the mains, the exhaust steam would enter the calandria of the
first effect at around 116°C. Usually, the steam is superheated, and an
average of 30° C superheat is not objectionable.
Since it is not advisable to operate the last effect of the evaporator
station at less than 55° C because of the poor heat transfer with high
viscosity syrups encountered at lower temperatures, the total temperature
drop across the station with saturated steam would thus be 61°C (116°C 55° C). With a quintuple effect, this corresponds to an average drop of
12°C across each body, a value that is about the minimum for satisfactory
heat transfer, so six effects would not be desirable. The heat transfer
would be better in a quadruple effect with an average temperature
difference of 15° C, which is about optimum. The total effect of
temperature increase in the juice caused by hydrostatic head also
increases with number of effects making the average temperature
difference smaller.
The sum of all these factors leads to the generalization that converting a quadruple effect to a quintuple effect by addition of a body of
the same heating surface does not raise the capacity of the evaporator
unless the initial steam pressure is increased. A fifth body added at the
front of the evaporator, and having a larger heating surface, would
increase the capacity if the vapor produced by the added area is used for
juice heating or pan boiling. The increase in capacity would be about
equal to the quantity so used.
In addition to temperature difference, heat transfer rate is proportional to the area of the tubes (heating surface), to the heat transfer
coefficient and thickness of the tubes. The latter two have little import
because with use scaling becomes the governing factor.
The capacity of the evaporators can be increased by installing a juice
preheater as an auxiliary ahead of the first effect. The heater, operating
on exhaust steam, should raise the juice temperature close to the flash
point.
VAPOR BLEEDING
As vacuum pans are single effect evaporating bodies, better steam
efficiency can be obtained by heating them with vapor from one of the
evaporator effects. The steam saving achieved increases with the
downstream position of the effect from which vapor is bled, as
represented in the formula:
Steam saving =
73
Where m is the position of the effect and η is the number of effects.
Thus, bleeding from the number one effect of"a quadruple would result in
a saving of one fourth of the .weight of vapor withdrawn.
It is generally desirable to maintain a positive pressure on the
calandria of vacuum pans, so normal practice is to heat pans with vapor
from the first effect. The same is true of secondary juice heaters where
juice must be heated above the atmospheric flash point. Primary juice
heating may be supplied by vapor from the second effect or, in some
cases, the third effect. Vapor bleeding also reduces the quantity of
condenser water required.
CAPACITY
The capacity of an evaporator station to remove water is established
by the evaporation rate per unit area of heating surface, the number of
effects and the location and amount of vapor bleeding. With no vapor
bleeding, the capacity is determined by the performance of the least
productive effect. The system is self-balancing. If a downstream effect
cannot use all of the vapor developed by the preceding effect, the
pressure in the preceding effect builds up and evaporation slows down
until equilibrium is established. With effects of uniform heating surface,
the productivity of the last effect is often the lowest on account of
heavier scaling and high syrup viscosity.
With vapor bleeding, the effect bled must have increased heating
surface to provide for the vapor withdrawn, as well as the amount
required for the downstream effects. Furthermore, all the bled vapor
must be withdrawn, otherwise, as explained above, the system will drop to
the evaporation level of the least productive effect. This means that the
bled vapor, if not used in the pans or heaters, must be vented if the
evaporation rate is to be maintained. This point must be carefully
considered in controlling the station.
If too much vapor is used by the pans and heaters, the pressure in the
vapor will drop and there will be insufficient vapor for them and for the
evaporators. Pressure can be maintained for the pans and heaters (vapor
loop) by throttling vapor to the downstream effects. This, however, will
reduce the total evaporation and in order to maintain evaporation, makeup
steam must be added to the loop.
OPERATION
In operating the station, the exhaust steam supply to the first cell is
controlled to give the total evaporation required to maintain the product
syrup at the set range of 65 to 70 refractometer solids. Uniform juice
feed is essential to good evaporator performance, especially with vapor
bleeding. Regulation can be maintained by level sensing in the evaporator
supply tank. Standard practice is to reduce the steam supply when juice
level drops in the evaporator supply tank. With a demand remaining for
first cell vapor, the vapor to the downstream cells is throttled. If the
supply tank level falls below a set minimum, water can be automatically
74
opened to maintain evaporator operation. If the first cell vapor pressure
drops too low, makeup with exhaust steam will be necessary in the vapor
loop.
The following factors
evaporator station.
are
important
in
maintaining
an
efficient
Automatic Control
Efficiency is enhanced by automatic
essential elements controlled are:
control instrumentation.
The
Absolute pressure
Syrup solids
Liquid level
Feed supply
Absolute pressure is controlled by regulating the quantity of water to
the condenser thereby maintaining a standard temperature of syrup in the
last cell of about 55° C. The absolute pressure set point will depend upon
the refractometer solids of the syrup. In the range of 65 to 70, the
absolute pressure will be around 10 cm of mercury (4.0 in.).
Syrup solids is controlled by regulation of the syrup discharge valve
from the last cell. A minimum of 65 refractometer solids is standard.
Concentrating above 70 should be avoided so that there is no danger of
crystallization occurring.
Liquid level in the tubes is an important factor in the heat transfer
rate. If the level is too low, all the tube heating surface is not used and
the tubes may dry out at the top. If the level is too high, the lower part
of the tubes will be flooded with juice moving at too low a velocity to give
maximum evaporation.
The optimum level is that at which the liquid is just being carried by
vapor bubbles to the top of the tubes with only a small flow over the tube
sheet. It varies with the size of the tubes, temperature, heat transfer
rate, scaling and viscosity of the juice. Under good conditions, the best
level is around 25% of the tube length. Rarely is it over 40%.
Since no flashing of feed occurs in the first cell, the situation differs
from the others. If the entering juice temperature is much below the
temperature of evaporation, the juice must be some distance up the tube
before it starts to boil, resulting in higher levels. In the last effect,
although the heat transfer rate is lower, the vapor expansion is much
greater because of the lower vacuum giving a higher explosive force up
the tubes.
Feed supply should be kept uniform using the supply tank
control. Dead band control is such that above a set level the
supply is signalled to cut back. Below a set level, the steam
reduced, and at a minimum level, a water valve is opened to
evaporator going.
for surge
source of
supply is
keep the
Condenser and Vacuum System
With
satisfactory
condenser
design
and
adequate
vacuum
pump
75
capacity, the points of concern in operation are the quantity of water,
water temperature and air leaks. Good condenser design will give the
rated capacity with a difference of 3°C between the condenser discharge
water and the entering vapor. The quantity of water required depends on
its temperature - the higher the temperature, the more water necessary.
Air leakage is usually the principal cause of evaporator malfunction.
Individual vessels and the piping system must be checked periodically for
leaks. Another common difficulty is air coming in with the juice feed.
This would not be detected in testing the equipment for leaks.
Condensate Removal
Inadequate condensate removal can cause partial flooding of the tubes
on the vapor side of the calandria with reduction of the effective heating
surface. Since the first effect is always under pressure, condensate is
readily removed by means of a steam trap. Condensates from the other
cells, which are under vacuum, are removed by pumping.
Condensates from individual cells are kept separate so that in case of
contamination only the water from the leaking unit is discarded. Also
condensate from the second cell contains volatile organic matter from the
juice in the first cell. The principal substance present is ethanol but other
alcohols, esters and acids are present and these are undesirable as
feedwater for high pressure boilers. This condensate is better used for the
house hot water supply than in the boilers.
Noncondensible Gas
A considerable quantity of noncondensible gas (air and carbon dioxide)
may enter the calandria with the heating vapor. Air enters also through
leaks in the vacuum vessels and carbon dioxide is generated in the juice.
If not removed, these will collect and interfere with the condensation of
steam on the tube surface.
Calandrias under pressure can be vented to atmosphere. Those under
vacuum must be vented to the vacuum system. Although it is easy to vent
to the body of the same effect, the gas is then transferred to the next
calandria and so on until removed in the condenser. Better practice is to
vent the effects individually to the vacuum system. This requires careful
valving to prevent appreciable vapor losses. A good guide is to monitor
the temperature of the vent line. The closer it approaches the calandria
temperature, the more vapor is lost. Calandria venting should be from
both the top and bottom of the calandria and at points opposite the vapor
entrance.
Scaling
Cane juice becomes saturated with respect to calcium sulfate and
silica before the concentration of dissolved solids reaches the desired
syrup level of 65 refractometer solids. The precipitation of these two
compounds, together with small amounts of other substances, causes a
build up of hard scale, principally in the last cell. Heat transfer is then
greatly impaired.
The quantity of scale deposited depends on the total concentration of
76
precipitable components in the juice, but the major constituent is calcium
sulfate. Its concentration can be reduced by using magnesium oxide instead of lime in clarification, since magnesium sulfate is soluble. A l though scaling cannot be prevented completely, control to workable levels
is possible by substituting magnesium oxide for part or all of the lime.
Scaling is greatest near the vapor entrance and at the bottom of the tubes.
Scale builds up with use and evaporation rate diminishes so that
cleaning becomes necessary. The most effective cleaning agent is caustic
soda which can be used directly as a 50% commercial solution. The
efficiency of the cleaning solution decreases rapidly with dilution so care
must be exercised to keep dilution to a minimum. Standard practice is to
spray the caustic soda over the tube sheet with sufficient steam in the
calandria to heat it. Treatment time averages two hours. The caustic
solution may be reused but becomes less effective because of dilution and
concentration of dissolved material. After caustic soda treatment, a
powdery film will remain on the tubes. This is easily removed by washing
with water followed by dilute sulfamic acid (0.25%) rinse and water wash.
Entrainment
Entrainment, or the carrying over of liquid with the vapor from one
effect to the calandria of the next or to the condenser of the final effect,
not only results in the loss of sugar but also causes contamination of
condensate for boiler feedwater and pollution of the condenser water
discharge from the factory.
Juice is expelled from the top of the tubes with a velocity sufficient to
atomize the liquid and project droplets to a considerable distance
vertically. The velocity increases from cell to cell reaching a maximum
in the final body where calculated velocities for 5 cm (2 in.) diameter
tubes may reach 18 m (60 ft) per second. This means that some droplets
have a velocity sufficient to project them 18 m. This is a hundred times
that in the first effect. The problem is, therefore, most serious in the last
effect and an efficient separator is essential. Separation, for the most
part, works by effecting the impingement of the droplets on a surface
from which the liquid can be returned to the cell.
The main problem encountered with separators, if their design is
adequate, is keeping them clean, as scaling takes place rapidly. Saddle or
ring-packed separators, approximately 30 cm (12 in.) in depth extending
across the top of the cell, are most effective provided they are also
cleaned when the evaporator is cleaned. This involves spraying the
caustic soda solution through the separator. Similarly, packed separators
in the vapor pipe need also to be cleaned with the evaporator. They are
less effective because of the high vapor velocity in the small diameter
pipe. They are better installed in the enlarged sections. Stainless steel
mesh separators extending across the body are effective if kept clean.
Zig-zag plate units, also placed across the body, are effective but more
expensive. With sufficient height above the calandria, simple flow
reversal baffle-type separators in the dome serve well.
Condensates should be monitored by conductivity routinely for entrainment. Condenser water should be tested for sugars on a scheduled basis.
77
Malfunctions
Evaporator problems arise from many causes. The principal ones are:
Low steam pressure
Air leaks in system
Condenser water supply
Vacuum pump
Noncondensible gas removal
Condensate removal
Scaling
Vapor bleeding
Difficulties with the steam supply, vacuum and water system are easily detected. Problems in individual cells with gas and condensate removal and scaling are best detected by observation of the temperature
drop across the cells. Temperature and pressure measurements in each
should be recorded on a regular basis. A malfunction can be traced by
changes in these measurements. For example, if the temperature gradient
across a cell increases, while the total drop across the set remains the
same, then the drop across the other cells will decrease. This means a
problem in the abnormal cell that requires investigation. It could be
caused by failure of condensate removal or noncondensible gas removal.
Of course, it could be caused by scaling, but this would occur gradually
and would only occur to a great extent in the last cell. A sudden decrease
in evaporation over the whole set might be caused by limited withdrawal
of vapor to heaters and pans. Unless vapor is vented, pressure will build
up in the vapor, reducing evaporation rate. This can be detected by
pressure readings.
EVAPORATOR CALCULATIONS
In the design of a new evaporator station, many assumptions are made
in order to arrive at the area of heating surface required in the multiple
cells. Often, the designer is given only the one figure - tons cane to be
handled per hour. He must, therefore, estimate the following:
Weight of juice to be evaporated per hour.
Refractometer solids content of juice.
Temperature of juice.
Exhaust steam pressure.
Exhaust steam temperatures.
Quantity of vapor to be bled.
Heat transfer rate at the heating surface of each cell.
Boiling point elevation of juice in each cell.
Refractometer solids of syrup.
Condenser water quantity and temperature.
Heat losses.
For an operator on a day-to-day basis, such estimations are of little
value. He is faced with conditions in which the quantity and composition
of the juice varies continuously, as does the exhaust steam pressure, the
requirements for bled vapor, heat transfer rate, and the quantity and
temperature of the condenser water. He can, however, make use of
78
approximations to arrive at problem areas in the evaporator set. Knowing
the exhaust steam and juice flow to the evaporator by measuring
temperatures, refractometer solids and pressure in the cells, calculations
may be made on the basis of assumed heat transfer values.
The Dessin-Coutanceau
tion is:
E
=
v
1
method is useful for this purpose. The equa-
(100 - B ) ( t v - tj)(tj - t c ) + ( t v - tj)
T670ÜÖ
Where
Ev
Β
tv
tj
tc
=
=
=
=
=
2
rate of evaporation in lb/ft /hr
average brix of juice in the cell
temperature of the vapor in ° F
average temperature of juice in ° F
temperature at vacuum in last cell in ° F
The constant of 16,000 applies for evaporators with tubes 1-3/4 χ 72
2
in., 5 lb/in. exhaust pressure and 26 in. Hg vacuum in the last effect.
The values from the last cell are set into the Dessin-Coutanceau equation
using measured values of solids, temperature of syrup, vapor to condenser
and calculated temperature of vapor to the cell. Calculations for the
other cells follow in order.
The following example is from an actual case of an evaporator showing
limited capacity.
Basis
To evaporate 157,400 kg (347,000 lb) water per hour from 196,860 kg
(434,000 lb) juice at 13.0 solids.
Quadruple effect with vapor bleeding from first cell.
Cell number
1
2
Heating surface, m
2
ft
1,394
15,000
2
Evaporation rate, kg/m /hr
2
lb/ft /hr
Vapor bled, kg/hr
lb/hr
39.0
8.0
2
3
1,324
14,250
1,231
13,250
25.4
5.2
4
27.3
5.6
975
10,500
34.2
7.0
22,680
50,000
Water evaporated, (without bleeding)
kg/hr
33,566
lb/hr
74,000
33,566
74,000
33,566
74,000
33,566
74,000
Total water evaporated, kg/hr
lb/hr
33,566
74,000
33,566
74,000
33,566
74,000
56,246
124,000
Solids out
18.3
24.1
35.3
65.0
Solids average
15.7
21.2
29.7
50.1
0.7
1.2
1.1
2.0
2.0
3.6
4.3
7.8
Boiling point elevation, ° C
°F
79
Vacuum
°F
mm Hg
Pressure
Cell 4
°C
in. Hg
Vapor t ( t c )
Juice t4 ( t j 4 )
tc - tj4
At (drop)
54.0
58.3
4.3
20.0
129.0
136.8
7.8
36.0
64.8
25.5
Vapor t4 ( t V 4 )
58.3 + 20
136.8 + 36.0
78.3
172.8
42.9
16.9
Juice t 3 (tj3>
78.3 + 2.0
172.8 + 3.6
80.3
176.4
tc - t j 3
80.3 - 54.0
176.4 - 129.0
At
26.3
8.3
47.4
15.0
Vapor tß ( t V 3 )
80.3 + 8.3
176.4 + 15.0
88.6
191.4
26.2
10.3
Juice t2 (tj2>
88.6 + 1.1
191.4 + 2.0
89.7
193.4
t c - tj2
89.7 - 54.0
193.4 - 129.0
At
35.7
5.8
64.4
10.5
Vapor t2 ( t V 2 )
89.7 + 5.8
193.4 + 10.5
95.5
203.9
11.4
4.5
Juice t\ ( t j i )
95.5 + 0.7
203.9 + 1.2
96.2
205.1
tc - tjl
96.2 - 54.0
205.1 - 129.0
At
42.2
6.9
76.1
12.5
Vapor t i ( t v i )
96.2 + 6.9
205.1 + 12.5
103.1
217.6
kg/cm
2
lb/in.
Cell 3
Cell 2
Cell 1
0.12
1.7
2
80
These calculations indicate that the evaporator should operate if the
2
2
exhaust pressure were 0.12 k g / c m (1.7 lb/in. ) and the exhaust steam
were supplied at the rate of 56,246 kg/hr (124,000 lb/hr). However, a very
high rate of evaporation was used for the fourth cell because of its small
2
2
heating surface - 34.2 kg/m /hr (7.0 lb/ft /hr). This is higher than
normally encountered by about 50%. The conclusion would be that the
evaporator cannot handle this quantity of juice. The limiting cell is
number 4, and it should be replaced with one having more heating
surface. A traditional heat balance can be developed at this point.
However, it must be kept in mind that conventional heat balances are
based on so many assumptions that they are of only limited value to the
operator. The proper approach is to measure the conditions in the
evaporator.
Because of the numerous conditions which cause an evaporator to
perform below calculated capacity, it is advisable to design with ample
provision to take care of all possible situations ranging from low steam
pressure to heavy scaling.
REFERENCES
1
Staub, S. and Paturau, M. Principles of Sugar Technology. P. Honig
(ed.), Elsevier, Amsterdam, 1963, Vol. Ill, p. 58.
81
Chapter 10
COMMERCIAL RAW SUGAR CRYSTALLIZATION
Raw sugar of commercial standards (Hawaii 1980) requires crystals of
reasonably uniform size 0.8 to 1.0 mm in length and 98.8 to 99.3 pol. Of
other quality factors, color, ash and filterability, only color remains of
major importance to the refiner. Marketable sugar meeting these
specifications can be produced efficiently by crystallization from syrup in
two steps. These are necessary because of the physical limits imposed by
the centrifugal separation of the sugar crystals from the mother liquor or
molasses. In the first step, A-massecuites utilizing syrup in the purity
range 83 to 88, when more than 60% of the sucrose is crystallized, the
massecuite becomes close to a solid mass which cannot be centrifuged.
Crystallization is therefore kept below this point, and the molasses is
returned for the second step or B-massecuites. In this crystallization, the
limit for the percentage of sucrose in crystal form is lower because of the
higher viscosity of the mother liquor, so that the maximum limit is not
over 50%.
The molasses from the B-massecuite is also returned for further
crystallization, but the product cannot be used as marketable sugar. This
C-massecuite sugar, or low grade, is too small in crystal size and too low
in pol and must, therefore, be dissolved and used in the feedstock for the
B-massecuite or, in a practice which is disappearing, as a magma of seed
crystal for the B-massecuite.
VACUUM PAN CRYSTALLIZATION
In generating sugar crystals of relatively uniform commercial size, use
is made of the fortuitous property of sucrose in forming extraordinarily
stable supersaturated solutions.
This reflects the reluctance of
concentrated sucrose solutions to develop crystal nuclei, a property
making it possible to establish a fixed degree of supersaturation in a
footing of sugar liquor. Then a predetermined number of nuclei, as seed
crystals, can be added, and by maintaining the supersaturation relatively
constant with simultaneous feeding of sugar liquor and by evaporating,
crystals of the average desired size can be grown.
Pan operations are normally conducted at a standard constant vacuum
of 125 mm Hg (5 in. absolute pressure).
A-Massecuite
A-massecuites are boiled on an initial footing of evaporator syrup.
Syrup purities will vary from time to time, the usual range being from 80
to 90 (refractometer pol purity). In order to have relatively uniform
conditions for seeding, it is generally advisable to establish a standard
purity of the graining charge - one that can be maintained most of the
82
time. A reasonable value in most locations is 85. In case the syrup is of
higher purity, Α-molasses is added to the footing syrup. Of course, if the
syrup purity is below 85, nothing can be done but adjust the standard
graining procedure.
The footing is then concentrated to the degree of supersaturation
optimum for establishing the grain. The degree of supersaturation can
only be sensed by indirect means. Those commonly used are:
Refractometer solids
Boiling point elevation
Electrical conductivity
Consistency
Refractometer Solids, as measured by a pan refractometer, is a simple
method of estimating the supersaturation. Data are available in the
literature showing the effect of temperature and purity on the
1
refractometer readings at various supersaturations (Figs. 10-1 and 10-2).
62
64
66
68
70
72
TEMPERATURE °C
74
76
78
Fig. 10-1. Purity-refractometer solids relative to
temperature at saturation.
80
83
TEMPERATURE ° C
Fig. 10-2.
Purity-refractometer solids relative to
temperature at 1.2 supersaturation.
Since the refractive index is highly temperature
temperature correction or compensation is essential.
sensitive,
good
Boiling Point Elevation (ΒΡΕ) is difficult to measure and for that
reason is the least used method. Proper instrumentation requires a
sensing of the vapor temperature at the surface of the boiling massecuite
compared with the temperature of boiling water at the same absolute
pressure. Literature data relating ΒΡΕ to temperature and purity are
available, however, and make this a useful technique (Fig. 10-3).
Electrical Conductivity is simple and is reasonably self-compensating
with regard to temperature. There is a problem in locating the electrodes
and keeping them free from scale. Its sensitivity to ash content makes it
difficult to use in areas where there are wide variations in the inorganic
salt content of the incoming juice.
84
Fig. 10-3.
Nomograph of boiling point, refractometer solids and purity
relationships at 1.3 supersaturation.
Consistency, up to the time of seeding, is a measurement of viscosity.
Like conductivity, it is somewhat temperature self-compensating. Since
the sensing rotor of a consistency probe is affected by the quantity of
vapor bubbles if placed above the tube sheet and by the movement
produced by a mechanical circulator if placed below the tube sheet, a
suitable location is difficult to find. A serious disadvantage of the method
is that viscosity is increased greatly by small quantities of dextrans and
gums, and a false indication of supersaturation is given when deteriorated
cane is being processed.
In choosing pan instrumentation for seeding, it would be desirable to
make a selection that could be used also after the grain is established.
The possible choices are electrical conductivity and consistency, since
refractometer solids and boiling point elevation both measure only the
molasses and do not make necessary allowance for the effect of the
crystals which is governing near the end of the strike. Both electrical
85
conductivity and consistency are sensitive to the crystal content, but they
suffer the drawbacks described.
The conclusion is that it is better to use one type of instrumentation
for seeding and another type for the remainder of the strike. An effective
system is the use of refractometer solids for seeding and consistency for
the remainder of the boiling. Regardless of the type of instrumentation
adopted, empirical relationships between the apparent supersaturation and
the instrument readout must be established under the conditions
obtaining. Considerable trial and error experience is thus essential.
In addition to sensing and control instruments for supersaturation, it is
important to have automatic absolute pressure control in order to
maintain stable temperature conditions. Massecuite level sensing is useful
for programming the changes inherent as crystallization continues. These
include purity decrease of the mother liquor, which means an increase in
viscosity, increase in crystal content and effect of the massecuite
pressure head. Automatic steam pressure control is advisable also.
The supersaturation at seeding point for an 85 purity footing should not
exceed 1.15 to be in a zone safe from spontaneous nucleation. This
corresponds to about 80 refractometer solids. A t this reading, the seeding
slurry is added. This is prepared by grinding good quality sugar in
isopropyl alcohol in a ball mill to a uniform size (see Chap. 12). The
amount of crystals necessary for the completed strike can be calculated
as shown in the example at the end of this chapter. Experience has shown,
however, that 2 to 3 times the calculated amount must be used to give the
right number of crystals. The reasons for this are multiple. The average
size of the seed particles made by the method shown is 4.5 microns.
Particles so small tend to conglomerate on standing. Another factor
seriously reducing the effective number of particles is the flash that
occurs when the alcohol slurry is introduced well above its boiling point.
In the flash, some of the particles cake together; others are lost by
sticking to the tubes and to the sides of the pan. Finally, some are
dissolved when they reach superheated zones near the tube walls.
The correct amount of seed must therefore be determined experimentally. It will vary with the type of pan and the manner in which the
pan is operated. Once the correct quantity is determined, however, the
procedure can be standardized.
After seeding, the steam to the calandria should be shut off while the
grain xbecomes established. This takes about 5 minutes, which allows the
introduced crystals to reach a size where they have enough surface to
absorb the sucrose introduced when the syrup feed is started. After the
waiting period, steam flow is resumed but only at a reduced rate. A t this
point water may be introduced for a short time to insure against reaching
too high a supersaturation. When the crystals have grown to a size that
they appear close together, automatic syrup feed can be started. This is a
critical time and feeding must not be forced at a rate faster than the
crystals can accept the sucrose, or new crystals may develop.
Feeding is continued until the pan is full (1.5 m above the tube sheet).
Massecuite is then cut to the finishing pans or a massecuite holding vessel.
86
The final sugar crystals will never be of uniform size, not only because
the original ground seed particles lack uniformity, but also because
apparently uniform crystals grow at different rates. This is caused by
non-uniform conditions in the pan and also by the fact that crystals with
molecular dislocations grow at a faster rate than those without. Fast
growing crystals tend to be more uniform in size than slow growing ones
for this reason.
Also, regardless of how carefully conditions are controlled, there will
always be some new nuclei formed in the boiling at all stages and stray
crystals will appear. The best that can be done is to keep the quantity of
these to a minimum. Good circulation is the major factor in preventing a
large number of stray crystals.
Normally, the volume of the massecuite at striking time must be a
minimum of 8 times the volume at seeding to give crystals of average
standard size. When the crystal size is suitable, feed is stopped and
evaporation is permitted to continue until the correct solids content for
centrifuging is reached. The refractometer solids at this point is
approximately 92.5. The massecuite should have a crystal content above
50%. With a corresponding pan drop of close to 20 points, the A-molasses
from the centrifugals will be in the range of 65 purity.
Since remelt (dissolved C-sugar) is a mixture of relatively pure sucrose
crystals and final molasses, it should not be used in the Α-strikes but
should be sent to the B-strikes. In this manner about 50% of the total
commercial sugar is not subject to crystallization in the presence of final
molasses. Quality, in particular color, should therefore be better.
B-Massecuite
The purity at which B-massecuites are boiled depends on the purity of
the Α-molasses and remelt. With an Α-molasses purity near 65 and remelt
in the 80 to 85 range, the B-massecuite purity can be at the 75 level. A
suitable footing is usually remelt.
Pan procedures are similar to those for Α-strikes. Viscosity is greater
and rate of crystallization is slower so more time is required. Because of
the lower purity, crystal content will be less and a target of 40% is
reasonable. Refractometer solids at the end of the strike should be at the
93 level.
Crystal Content and Pan Drops
It is important to obtain the highest yield practicable at each stage of
the crystallization process. The yield is the percent of the sucrose in the
massecuite which is in crystal form, as shown by the formula:
massecuite purity - molasses purity
100 - molasses purity
m
a
s
s
e
c
u ei
f r a c to m e t e r solids
tr e
Massecuite purity minus molasses purity is usually called pan drop. This
figure is the one commonly observed in comparing pan work, but the
crystal content is a more useful value because of the sensitivity of yield
to purity levels. For example, compare the crystal content at the same
pan drop (20) for massecuites of 80 and 85 purity.
87
Crystal content
80 Purity =
85 Purity =
?n
— — χ 92.5 =
100 - 60
— — χ 92.5 =
100-65
46.3
52.9
So the higher the purity the less the pan drop that is necessary to obtain
the same yield.
As a rough guide, it is not necessary to multiply by the refractometer
solids so relative yields may be noted from the ratio of the pan drop to
100 minus molasses purity.
Vacuum Pan Design
For efficient operation, the vacuum pan must be designed for good
circulation, essential to both rapid and uniform crystallization. In
calandria pans, natural circulation by expulsion of the massecuite upward
through the tubes by the action of expanding bubbles of vapor, and
drainage down through the center well give effective distribution in a
well-designed vessel. Circulation can be improved, nevertheless, by
means of a mechanical circulator at the bottom of the center well.
The critical points in design are short tubes, low head of massecuite
above the tube sheet at time of striking, minimum volume between the
bottom and the tube sheet, belt diameter the same as the calandria and
proper heating surface in the tubes.
Optimum length for standard 10 cm (4 in.) diameter tubes is 75 cm (30
in.). In longer tubes, the percolation effect is diminished. Shorter tubes
do not provide enough heating surface. The tubes should be arranged
around a center well of approximately 40% the diameter of the pan.
Circulation diminishes as the level of massecuite above the tube
increases, so the level at the time of striking should not be over 1.5 m (5
ft). This volume capacity should not be obtained by flaring the pan to a
greater diameter above the tube sheet (so-called low head design) because
the inventory of massecuite beyond the tube sheet suffers from poor
circulation. The belt, therefore, should be the same diameter as the
calandria.
The pan bottom should be streamlined so that the massecuite flow is
directed from the center well outward with a minimum volume of
massecuite below the tubes.
The necessary heating surface of the tubes depends to some extent on
the pressure of the heating vapor used. In pans boiled on vapor from the
2
2
first evaporator effect, the pressure is a normal 0.5 k g / c m (7 lb/in. ). A
good design would, in this case, provide a ratio of heating surface to
2
3
2
3
volume of 5 m / m (1.5 f t / f t ) .
The location for a mechanical circulator is on the center line of the
bottom tube sheet. The design should not be that of a ship's propeller to
give forward thrust but to force the massecuite laterally under the tubes.
88
The speed depends upon the diameter but must be left low, 40 rpm is
3
optimum for a 600 hi (2000 f t ) pan.
Procedures
Instrumentation - Semiautomatic control - Graining by refractometer Boiling by consistency or conductivity
1. Automatic absolute
condenser. Recorder.
2.
pressure
control
by
flow
of
water
to
Pan refractometer. Recorder.
3. Automatic feed control, sensing by means of consistency or
electrical conductivity to maintain maximum supersaturation. Recorder.
4.
Automatic level control. Recorder.
5.
Pan vapor thermometer. Indicator.
6.
Pan microscope.
7.
Pressure gauge calandria. Indicator.
Operation - typical example
1. Draw in footing of just sufficient volume to cover the tube sheet
when evaporated to the seeding point. Footing should be syrup if its
purity is 85 or below. If syrup purity is above 85, add enough A-molasses
to lower the footing purity to 85.
2.
sure.
Concentrate to 80 refractometer solids, at 125 mm absolute pres-
3.
Shut steam valve. Keep mechanical circulator running.
4.
Add 30 ml slurry per 283 hi (1000 f t ) final massecuite.
3
5. When grain is established (5 min) open steam valve half normal
and start syrup feed.
6. After 10 minutes, open steam valve to normal and maintain supersaturation of 1.2 until massecuite is 1.5 m above the tube sheet.
7.
Cut seed to striking pans or seed storage.
8. Continue boiling in striking pans until crystal reaches average size
of 0.8 mm feeding Α-strikes with syrup and B-strikes with Α-molasses and
remelt. If crystals have not reached 0.8 mm when pan volume reaches 1.5
m above the tube sheet, another cut must be made.
9. Drop strikes at 92.5 and 93.0 refractometer solids, respectively,
for the A - and B-strikes.
Calculation
3
Seed slurry required for 283 hi (1000 f t ) massecuite.
Massecuite:
3
283 hi (1000 f t )
93 refractometer solids
89
Crystal content 55% refractometer solids
Crystal size 0.8 mm
Weight of massecuite at 1.4 kg/1 = 39,620 kg
Weight of crystal in massecuite = 20,265 kg
39,620 χ 0.93 χ 0.55
Seed required:
20,265 χ (M045)3
\ 0.8 '
20,265 χ 0.0000001778 = 0.0036 kg or 3.6 g
Slurry has 1000 g in 2 liters alcohol
Total volume = 2630 ml
1000
·+ 2000 = 630 + 2000
1.587
lmlhas
3
S - S
=0
38
6
'
= 9.5 ml
0.38
Estimated practical quantity is 3 times this or 28.5 ml (round number 30).
Quantity of Massecuites
The quantity of massecuites boiled is a minimum in a straight-forward
boiling system with the only return being C-sugar to the B-massecuites.
Any inboiling, that is, return of low purity molasses to higher purity
massecuites, increases the total.
The quantity also becomes greater as the syrup purity decreases. The
following calculations illustrate this effect in examples of 85 and 80
purity syrups. For simplicity, the sugar has been calculated at 100 pol and
no losses in processing are included. Straight A , B, C boiling is used with
C-sugar returned to B-massecuites. Basic assumptions include:
85 Purity syrup
80 Purity syrup
A-Massecuite
crystal yield
C-Massecuite
purity
50
45
58
56
85 Purity syrup
Tons
Syrup
Refractometer solids
Pol
Nonsugars
100.00
65.00
55.25
9.75
Final molasses
purity
35
36
90
A-Massecuite
Crystal yield 50% assumed
Sugar crystal, tons = 27.63
0.5 χ 55.25
Refractometer solids to B-massecuite, tons = 37.37
65.00 - 27.63
Α-molasses purity = 70
85 - purity
u
100 - purity
'°
C-Massecuite
Assume:
58 purity massecuite
35 purity final molasses
Crystal yield = 35%
ό
100 - 35
"' *
Nonsugars, tons =9.75
Pol, tons = 13.46
Pol
- π c
o
5 8
P o l + 9.75 " ° Refractometer solids, tons = 23.21
9.75 + 13.46
Sugar crystal, tons = 4.71
0.35 χ 13.46
Sugar (solids) at 80 purity
returned to B-massecuite, tons = 6.81
4.71
0.692
100
\
35
/
80
45
/
\
20
x
1
00 =
6
20 χ
1
00 =
3
ët
9
·
2 %
0
%
c r
Y
m
o
s t al
l
a
s
solids
ss e
s o l i sd
D D
Molasses (solids) returned to B-massecuite, tons = 2.10
6.81 - 4.71
Pol returned to B-massecuite, tons = 5.45
4.71 + (0.35 χ 2.10)
Final molasses
Nonsugars, tons = 9.75
Pol, tons = 5.25
_P£l
= 0 35
U e DJ
Pol + 9.75
Refractometer solids, tons = 15.00
9.75 + 5.25
91
B-Massecuite
Tons
Refractometer solids from A-massecuite
Refractometer solids from C-massecuite
Total
Pol from A-massecuite
Pol from C-massecuite
37.37
6.81
44.18
27.63
5.45
Total
33.08
Purity = 74.9
33.08
χ 100
44.18
40.2%
Crystal yield
74.9 - 58
0.402
100 - 5 8
Quantity of massecuite
Tons refractometer solids
A-massecuite
B-massecuite
C-massecuite
65.00
44.18
23.21
Total
132.39
80 Purity syrup
Tons
Syrup
Refractometer solids
Pol
Nonsugars
100.00
65.00
52.00
13.00
A-Massecuite
Crystal yield 45% assumed
Sugar crystal, tons = 23.40
0.45 χ 52.00
Refractometer solids to B-massecuite, tons = 41.60
65.00 - 23.40
Pol to B-massecuite, tons = 28.60
52.00 - 23.40
Α-molasses purity = 63.6
80 - purity
0.45
100 - purity
C-Massecuite
Assume
56 purity massecuite
36 purity final molasses
Crystal yield = 31.3%
56 - 36
= 0.313
100 - 36
92
Nonsugars, tons = 13.00
Pol, tons = 16.55
Pol
ηcß
5 6
Pol+13.00 - ° Refractometer solids, tons = 29.55
13.00 + 16.55
Sugar crystal, tons = 5.18
0.313 χ 16.55
Sugar (solids) at 80 purity
returned to B-massecuite, tons = 7.53
5.18
0.688
100
Ν
36
44
Ü
44
80
x
1
00 =
6
8
·
8 %
c r
y
s t al
s o l l sd
/
~
| ^ χ 100 = 31.2% molasses solids
Molasses (solids) returned to B-massecuite, tons = 2.35
7.53 - 5.18
Pol returned to B-massecuite, tons = 6.03
5.18 + (0.36 χ 2.35)
Final molasses
Nonsugars, tons = 13.00
Pol, tons = 7.31
=
3 6
Pol+13.00
°"
Refractometer solids, tons = 20.31
13.00 + 7.31
B-Massecuite
Tons
Refractometer solids from A-massecuite
Refractometer solids from C-massecuite
Total
Pol from A-massecuite
Pol from C-massecuite
Crystal yield
i l d = 33.0%
70.5 - 56
= 0.330
100 - 5 6
49.13
28.60
6.03
Total
Purity = 70.5
34.63
χ 100
49.13
41.60
7.53
34.63
93
Quantity of Massecuite
Tons refractometer solids
A-massecuite
B-massecuite
Omassecuite
65.00
49.13
29.55
Total
143.68
These calculations show that with 85 purity syrup the quantity of solids
handled is about double that incoming with the syrup. A t 80 purity it is of
the order of 10% additional. Most of the increase is on the C-massecuite
where the solids handled is greater by 27% and the final molasses is
greater by 35%.
REFERENCES
1
Honig, P. (ed.), Principles of Sugar Technology, Vol. 2, Elsevier,
Amsterdam, 1959, pp. 358-359 (from Thieme).
95
Chapter 11
LOW GRADE SUGAR CRYSTALLIZATION
Traditionally, the final step in sugar recovery is allowed to take place
by cooling in crystallizers rather than during evaporation in vacuum pans.
The reason for this is that the rate of crystallization becomes
progressively slower as the molasses purity falls. A more cost effective
procedure, therefore, is to boil low grade massecuite in a vacuum pan for
a limited period of time, then discharge it into atmospheric crystallizers
where sugar can be allowed to crystallize at length without evaporation.
CRYSTALLIZATION BY COOLING
The technology of crystallization by cooling differs markedly from
that of crystallization by evaporation, and although crystallizers appear to
require little attention, success in recovery depends to a great degree on
the details of their operation. Two major factors are important in the
technique.
1. A t a temperature range of 50 to 60° C and below, the rate of
diffusion of sucrose molecules to the surface of a crystal exceeds the rate
of deposition on the crystal; whereas at higher temperatures, the rate of
diffusion is less than the rate of deposition (Dedek).
2. In this same temperature range (50-60° C), saturated molasses (and
1
supersaturated molasses also) has a minimum viscosity (Fig. 11-1 J.
Application of these
following conclusions:
facts
to crystallizer operation leads to
the
1. Mixing in the crystallizer is necessary only to prevent the crystals
from settling and to aid in heat transfer.
2.
Maximum crystal surface
sucrose deposition.
3.
is essential
to a favorable rate of
The optimum holding temperature is in the range of 50 to 60° C.
These points must be superimposed upon the one basic fact that water
is the principal component that keeps sucrose in solution. A t a given
temperature, the saturated molasses purity varies directly with the water
content. In order to obtain a lower purity molasses water must be
removed, increasing the saturation temperature. But as water is removed
molasses viscosity rises sharply, giving proportionately higher massecuite
consistency. As the crystal content is relatively constant, molasses
viscosity becomes the limiting factor. Since consistency must be kept at
a level that permits massecuite pumping and separation of crystals from
the molasses by centrifuging, molasses viscosity must be carefully
controlled.
96
Fig. 11-1.
Viscosity-temperature relationships for saturated molasses at
varying water content.
With the optimum viscosity-saturation relationship occurring in the 50
to 60° C temperature range, the water content of the massecuite should be
targeted to give the maximum consistency of massecuite practicable.
Such a practice will give the lowest attainable molasses purity for a given
material composition in a fixed period of time. The control procedure for
the crystallizers, therefore, is determined by the composition and physical
character of the massecuite discharged from the pans.
Crystallizer Design
Design criteria for a crystallizer are directed toward heat transfer
from and to the viscous mixture of sugar crystal and molasses. The most
97
effective design is that of rotating pipe coils through which water is
circulated. These give enough movement to prevent crystals from settling
and provide reasonable opportunity for heat transfer. They are more
effective for heating than for cooling. When the coils are hotter than the
massecuite, the molasses viscosity is less and the massecuite slides off the
surface permitting access of cooler material. When the coils are cooler
than the massecuite, the cooled material at the surface of the coil
becomes more viscous and clings to the coil insulating it from the body of
the massecuite. For this reason plate-type crystallizers are not effective
for cooling heavy massecuite.
The coils must be far enough apart that massecuite is not carried as a
body between them. The coils must move through the massecuite giving
time for exposing new material to them. The coils must be heavy and well
supported.
Since little agitation is required, a speed of 12 rph is sufficient.
Massecuite level should be above that of the coils to prevent incorporation
of air which causes an increase in the viscosity of the massecuite.
Operation
Procedure
Instrumentation.
1.
2.
Thermometers. Recorder.
Drive motor ammeter. Alarm.
Batch crystallizers.
1.
Cool the massecuite to the range 50-55° C. In an efficient
water-cooled crystallizer, this will take some 15 hours. With
cooling water at ambient temperature, it is practically
impossible to cool a properly boiled massecuite rapidly
enough to cause spontaneous new grain.
2.
Hold the massecuite at the chosen temperature by circulation
of water at a slightly higher temperature for a minimum of
15 more hours. After this period, the supersaturation and
molasses purity have become so low that the crystallization
rate is exceedingly slow. It is, therefore, of little practical
value to hold longer, although it can be done if capacity is
available.
3.
Start reheating in the crystallizer by circulation of about
60°C water. This can be done for a variable period of time
depending upon the need for the crystallizer for subsequent
strikes and the availability of massecuite heaters. The object
of reheating is to reduce the supersaturation to close to zero
before centrifuging. With the use of massecuite heaters,
reheating in the crystallizer can be kept to a minimum
sufficient to improve the flow of massecuite from the
crystallizer. Otherwise, several hours reheating will be
necessary. The temperature to which massecuite is reheated,
either in the crystallizer or heater, will depend upon the
98
saturation temperature of the molasses. Since it is not
practical to determine the saturation temperature on each
strike of massecuite, a general temperature is chosen, based
upon
periodic measurements
on typical samples and
experience.
Continuous crystallizers
The same principles hold for the operation of continuous
crystallizers with necessary variations in procedure required by the
individual installation. For continuous operation, the massecuite must be
discharged from the pan into a strike receiver which acts as a reservoir to
feed the bank of crystallizers.
The preferred number of units in a bank is 7, permitting 2 or 3 for
cooling, 2 or 3 for holding and 2 for reheating. In order to make coil
repair possible without emptying two units, flow is from the top at one
end, out the top of the other. Surface flow is prevented by a horizontal
baffle plate located one third of the way from the entry point and
extending not more than 50% of the depth of the massecuite. Troughs
between the units should be wide and shallow to give a channel rather than
a tube-like flow.
A possible water flow diagram is shown in Figure 11-2. Cooling
water, at the chosen temperature, flows through the second crystallizer
and then through the first. The reason for counter current flow is to
avoid, to some extent, too rapid initial cooling causing solidification
around the coils and bypassing of massecuite. Heating water enters the
last body and flows through the next to last, then out to a tank where it
mixes with cooling water and is temperature controlled for holding water
which goes through the third, fourth and fifth bodies in series.
Temperature control is required only at two points - the holding water and
the heating water. Quantity of water must be controlled relative to the
flow of massecuite.
In the operation of continuous crystallizers it is important to keep
in mind that the retention time is a function of the rate of flow, which is
set by the quantity of massecuite that must be handled per unit of time.
The quantity of low grade massecuite is governed largely by the purity of
the syrup. As an example of the wide variation, the quantity produced
from 80 purity syrup would be about double that at 88 purity per unit of
pol in the syrup. Therefore, unless the rate is controlled, the retention
time with massecuite from 80 purity syrup would be only half that from
88. For optimum operation, therefore, the flow through the continuous
system must be monitored and controlled.
VACUUM PAN CRYSTALLIZATION
Low grade boiling procedures are set by the crystallizer operation
discussed above, the objective being to provide a massecuite of a
consistency and purity which will give maximum recovery in the
crystallizers.
99
Ο m
LÜ
U
Ο
LÜ
3
Lü
Hl
<
5
ο
χ
CO
(Τ
Ο
ΙΟ
"Mr
(Τ
LÜ
*<
ο
Ο Ο
If)
C
M
rÜJ
ϋ Ο
LÜ
ω
<
^
Γ
λα
ζ *
— ο
ζ
_ι
ο
ο
ο
Fig. 11-2. Continuous crystallizer bank.
ΓΟ
tr
LU
Η
<
100
Consistency is governed by the viscosity of the molasses, crystal
content and the size of the crystals. Because of the slow rate of growth,
the size of the crystals is usually limited to the range of 0.20 to 0.30 mm,
with workable average being 0.25 mm. The crystal content, which is the
percent of the total solids in crystal form, has a practicable range of 25 to
40, depending upon the purity of the massecuite.
The viscosity of the molasses, at saturation, depends upon the quantity
and nature of the nonsucrose matter and the temperature. Temperature,
therefore, is the only controllable variable. Since the minimum viscosity
is in the neighborhood of 55° C, the molasses should be concentrated in the
pan to a water content that will make it saturated at 55° to 58° C.
Because the composition of the raw material (mixed juice) may vary
from hour to hour, field to field, and week to week, it is not possible to
maintain a constant quality massecuite by control of pan work.
Experience alone will enable a pan operator to judge at striking time,
70°C and unknown supersaturation, what the consistency of a massecuite
will be many hours later at 50° C and minimal supersaturation. He will,
therefore, try to have the solids content at an average as high as possible
to give a massecuite that can be handled in the crystallizers, pumps,
heater and centrifugals.
Low grade vacuum pan work should thus be directed to giving an
average massecuite at the time of centrifuging of the following
composition:
Purity
Crystal content (refractometer solids)
Crystal size
Molasses, saturated at
58
30-35%
0.25 mm
55°-58° C
The purity at which the massecuite is boiled is dependent to a major
extent on the purity of the B-molasses. Starting with syrup purity of 85,
with standard pan drops on A and Β strikes, the B-molasses purity should
not be higher than 56. This permits a massecuite purity of around 58.
In order to give the seed a fast start, the footing should be a few
points higher than the final massecuite. In a fully counter current system,
this should be made up of B-molasses and remelt. As it is inconvenient to
make this mixture, Α-molasses is often used because it is about the right
purity.
The footing should be concentrated to a supersaturation of 1.2, then
the estimated quantity of seed slurry is added (see Table 11-1). With a
mechanical circulator to maintain good movement in the pan, the steam
should be shut off while the seed is becoming established. Otherwise
there is danger of concentrating the footing so much that spontaneous
nuclei may be formed. After a waiting period of about 10 minutes, steam
can be opened. A t this point water may be introduced until vigorous
boiling results before starting B-molasses feed.
Automatic feed
control can be programmed to allow
the
supersaturation to be held as high as possible, governed by the consistency
101
Table 11-1
MOLASSES
Refractometer Solids
Supersaturation 1.20
Refractometer Pol Purity
Tempérât ure
U
C
60
62
64
66
144
62.3
83.5
83.2
82.8
82.5
146
63.4
83.8
83.5
83.2
82.5
148
64.5
83.9
83.6
83.3
83.0
150
65.5
84.1
83.8
83.5
83.1
152
66.8
84.2
83.9
83.6
83.3
154
67.9
84.3
84.0
83.8
83.5
156
69.0
84.5
84.2
83.9
83.6
158
70.1
84.6
84.4
84.1
83.8
160
71.2
84.8
84.5
84.2
83.9
162
72.3
84.9
84.7
84.4
84.1
164
73.5
85.1
84.8
84.5
84.3
166
74.6
85.3
85.0
84.7
84.4
168
75.7
85.6
85.3
85.0
84.7
170
76.8
85.7
85.4
85.2
84.9
which should never be allowed to rise above the point of good circulation
and vigorous boiling. Particularly toward the end, when the massecuite
level is high, circulation becomes sluggish. Water addition is often
advisable to improve the circulation by ebullition.
During all boiling, the absolute pressure should be kept constant at
about 125 mm Hg. (5 in. absolute pressure). Higher temperature improves
circulation and speeds crystallization. Temperatures above 70° C are not
advisable, however, because of excessive inversion and propagation of
reactions of reducing sugars and amino acids which cause excessive
frothing in crystallizers and final molasses.
When pan capacity is reached, which should not give a massecuite level
in excess of 1.5 m above the tube sheet, the massecuite is cut to finishing
pans. There boiling is continued until the crystals are of sufficient size
and the pan drop target is reached. A t striking time, a molasses purity of
42 would mean a pan drop of 16 points. This would give a crystal content
of 26%. The average size of the crystals should be larger than 0.2 mm.
102
Vacuum Pan Design
Pans for low grade boiling are basically the same as for A and
strikes. The only difference is that because of the much slower rate
crystallization the heating surface does not have to be as large.
2
3
2
3
nominal ratio of heating surface to volume of 4 m / m (1.2 f t / f t )
standard.
Β
of
A
is
Mechanical circulators should run at a lower speed because of the
higher viscosity massecuites.
Operation
Procedures
Instrumentation (semi-automatic control)
1.
2.
3.
4.
5.
6.
Automatic absolute pressure control. Recorder.
Pan refractometer. Recorder.
Automatic feed control, sensing by means of consistency or
electrical conductivity. Recorder.
Automatic level control. Recorder.
Pan vapor thermometer. Indicator.
Pan microscope.
7.
Pressure gauge, calandria. Indicator.
Typical Example
1. Draw in footing (B-molasses plus remelt or A-molasses)
sufficient in volume to just cover the tube sheet when
evaporated to the seeding point. Purity of footing 65.
2. Concentrate rapidly to 84 refractometer solids at 125 mm
absolute pressure.
3. Shut steam valve. Keep mechanical circulator running.
3
4. Add 600 ml seed slurry per 283 hi (1000 f t ) final massecuite.
5. When grain is established (10 min.), open steam valve and
start B-molasses feed. (B-molasses purity is estimated to be
56.)
6. Maintain maximum supersaturation consistent with vigorous
boiling (1.3) until massecuite is 1.5 m above tube sheet. If
boiling is not vigorous near the end, feed sufficient water to
maintain it.
7. Cut seed to striking pans or seed storage.
8. Continue boiling in striking pans until crystal reaches the
minimum size of 0.2 mm.
9. Drop strikes at a refractometer solids that will give a
saturated molasses at 55°-58°C (95 massecuite refractometer
solids normally).
Calculations
3
Seed Slurry Required for 283 hi (1000 f t ) Massecuite
Massecuite
3
283 hi (1000 f t )
95 refractometer solids
103
Crystal content 35% of refractometer solids
Crystal size 0.25 mm
Weight of massecuite at 1.5 kg/1 = 42,450 kg
Weight of crystal in massecuite = 42,450 χ 0.95 χ 0.35 = 14,115 kg
Seed Required (theoretical)
0
4 5
3
0.0000058 = 0.082 kg or 82.0 g
14,115 χ ( ° ό ' 2 ° 5 ) = 14,115 χ Ο.ι
Slurry has 1000 g in 2 liters alcohol.
Density of sugar = 1.587 g/ml
Total volume =
1 ml has
.'. Require
1000
+ 2000 = 630 + 2000 = 2630 ml
1.587
1000
= 0.38 g
2630
82.0
= 216 ml
0.38
Practical estimate = 3 χ 216 = 648 ml
Nominal value = 600 ml
REFERENCES
1
Micheli, L. I . A . and DeGyulay, O. S., Proc. Intern. Soc. Sugar Cane
Tech. 6, 1938, p. 1094.
105
Chapter 12
GENERALIZATIONS AND DATA ON SUCROSE CRYSTALLIZATION
GENERALIZATIONS
Temperature
A 10°C increase in temperature increases rate of crystallization 3.5
times.
Supersaturation
At 50° C, an increase in supersaturation from 1.05 to 1.15 increases the
rate of crystallization 5 times.
A change in refractometer solids of 1.0 changes supersaturation by
0.06.
Temperature and Supersaturation
Same rate of crystallization *
Temperature
Supersaturation
°C
70
60
50
40
1.25
1.30
1.35
1.40
Viscosity
A 5°C decrease in temperature doubles the viscosity of final molasses.
The viscosity of massecuites is 3 to 6 times that of the suspending
2
molasses. Also, the following are approximate estimates of the increment required of a variable to cause a 20% decrease in viscosity:
0.4% decrease in solids
2° C increase in temperature
3% decrease in crystal content
Threefold decrease in mean crystal size
10% increase in purity
100% increase in coefficient of variation of crystal size
106
Purity
Crystallization rate relative to pure
sucrose at 1.09 supersaturation
Purity
Relative rate
100
92
80
70
100
59
14
2
3
Recirculation of Final Molasses with Return of Low Grade Sugar
Sugar purity
Relative increase
over 90 purity
%
86
82
79
60
140
245
Low Grade Seeding
ml of seed slurry =
Seed Slurry
f
t 3
"assecuite
3
Preparation
1000 g sugar
2 liters isopropyl alcohol
Grind 24 hr in a jar mill of 4-liter capacity with 4 kg of 20 χ 20 mm
cylinders.
Average particles
Number:
Size:
4.8 χ 10^ particles/ml slurry
4.5 microns
Crystal Yield
Crystal
yield
% = ref. sol. mass, χ Pur. mass. - pur mol.
J
J
100 - pur. mol.
Exhaustion
Exhaustion is parts of sucrose in crystal per 100 parts sucrose in
massecuite.
Λ The crystal yield formula must be multiplied by
100
% pol in mass.
107
. „ . ,
ref. sol. mass. pur. mass. - pur,
mol.
c
. . Exhaustion -r-, Ί .
χ *•—
:
% pol in mass.
100 - pur. mol.
2
lOO
pur. mass.
x
pur, mass. - pur, mol.
100 - pur. mol.
Perk^ says this figure should be over 60 irrespective of the type of
massecuite.
DATA
Single Crystal Measurements
5
Screen opening
mm
Weight
mg
Surface
mm^
0.5
0.6
0.7
0.8
0.9
1.0
0.139
0.240
0.381
0.568
0.809
1.110
1.22
1.75
2.39
3.12
3.94
4.87
Crystal Growth Rate, mg/m^/min.G
\
Super-\
saturation\
1.005
1.010
1.015
1.020
1.025
1.030
1.035
1.040
1.045
Temperature ° C
0
20
30
40
50
60
5
40
80
120
75
150
225
145
285
490
240
490
800
340
720
1340
150
190
230
380
495
625
675
855
1060
1200
1800
2300
2210
3100
275
320
360
755
910
1115
1300
1540
1800
2870
3510
4060
420
480
525
1320
2085
2580
9
14
19
1.050
1.055
1.060
32
1.065
1.070
1.080
38
45
1.090
1.100
1.110
52
62
76
25
575
620
108
Fig. 12-1. Effect of purity on crystallization rate.
3
109
MASSECUITE
TOTAL-SOLIDS
SUCROSE
CRYSTAL
CONTENT
NON-SUCROSE
VISCOSITY
CIRCULATION
MOBILITY
SUPERSATURATION
TIME
CRYSTALLIZATION
RATE
MOLASSES
EXHAUSTION
Fig.
TEMPERATURE
12-2. Variables governing molasses exhaustion.
CRYSTAL
SURFACE A R E A
110
SYRUP
85
Β REMELT
90
C
MASSECUITE
76
MASSECUITE
88
CO
-Jin
- J 1^o
ο
Σ
m
5
COMMERCIAL
SUGAR
9 9 . 3 POL
MASSECUITE
60
Id
CO
CO
<
CO
to
CENTRIFUGAL
REMELT
85
Β
CENTRIFUGAL
CENTRIFUGAL
\ΛΛΑΛΑΑΛΑΛΛ
vyvWvWWT
ο
F I G U R E S ARE REFRACTOMETER
POL P U R I T I E S
Β
C REMELT
85
REMELT
90
FINAL
MOLASSES
35
Fig.
12-3. 3-Massecuite, 1-high pol sugar boiling system.
Ill
REFERENCES
1
2
3
4
5
6
7
Saint, John, Int. Sugar J., 35 (1933) 311.
Awang, M. and White, E. T., Proc. Queensland Soc. Sugar Cane Tech.,
43 (1976) 263.
1
unpublished data, Experiment Station of the Hawaiian Sugar Planters
Association.
Perk, C. G. M., The Manufacture of Sugar from Sugar Cane, Sugar
Milling Research Institute, Durban, 1973, p. 203.
Wieninger, J. Α . , Jahresbericht Zuckerforschungs Institute, 1974-75, 78.
Kucharenko, J. Α . , Planter and Sugar Manufacturer, 30 (1928) 485.
McGinnis, R. Α . , Sugar J., 39 (1976) 7.
113
Chapter 13
CENTRIFUGATION
Although the mechanics of the process run counter to apparently sound
principles, sugar crystals are universally separated from molasses by
centrifugation. Under centrifugal force the denser sugar is restricted
from flying away from the lighter liquid molasses by means of a
perforated screen. The molasses then finds its way through the spaces
between the crystals and out through the perforations.
The two types of machines in use are batch and continuous.
BATCH MACHINES
Commercial sugar
Standard machines for commercial sugar have 1220 mm χ 915 mm (48
in. χ 36 in.) baskets driven by single-winding, pole-changing, two-speed,
600/1200 rpm, alternating current induction motors. A t full speed and at
the center of the sugar cake, the nominal gravity factor is 900. A
separate reverse drive small motor is used for low speed discharge. Fully
automatic operation requires only the time settings for the successive
steps of initial acceleration, charging, low speed acceleration, washing,
high speed acceleration, running, regenerative braking, mechanical
braking and discharge.
These settings are determined by the
characteristics of the massecuite and the sugar quality desired. A
complete cycle normally takes about 3 minutes.
Higher capacity
machines, 1372 mm χ 1016 mm (54 in. χ 40 in.), are also available.
It is generally accepted that the separation of molasses takes place in
three stages. These are:
1. Removal of molasses in excess of that required to fill the space
between the crystals.
2.
Further expelling of molasses leaving voids between the crystals.
3.
Reducing the molasses film around the crystals.
The rate of removal of molasses is rapid at first and very slow later
on, governed by the size of the crystals and the viscosity of the molasses.
2
With crystals of uniform size the rate is proportional to m /n where m is
the length of the crystals and η the viscosity of the molasses. Rate of
molasses removal is less in sugars of nonuniform crystal size because of a
packing effect. In extreme cases small crystals can almost completely
block the passage of molasses, leaving a layer on the inside of the solid
mass of sugar.
114
Water washing is necessary to reduce the molasses film in order to
give the desired sugar pol. The effectiveness of the wash depends upon
the method of application, quality of the sugar with respect to size
distribution and form, quantity of water, temperature and timing of
application. The water should be distributed uniformly over the surface of
the sugar and should be hot (75° C or above). It should be applied at the
moment the excess molasses is gone from the crystal but the space
between is still filled with molasses (before the machine reaches top
speed). This can be observed as the point at which the face of the sugar in
the basket starts to lighten as the molasses leaves. The water then will
give a plug type flow and uniform washing. If applied after voids are
present between the crystals, then the water will seek the path of least
resistance and leave some sugar over washed and some underwashed.
The quantity of water and spinning time are adjusted to give the pol
desired. In producing high pol sugar (99.3-99.5), close to complete washing
is necessary. Two stages of application are effective in this case, the
first as described, and a second just when the machine reaches top speed.
The second wash quantity is adjusted to give the sugar pol required.
Double washing also makes possible the separation of the wash liquors.
Low Grade Sugar
The small-sized crystals and high viscosity of the molasses make
washing in batch centrifugals impracticable. Separation of molasses
therefore requires use of higher gravity factors and longer spinning times
than used for commercial sugars. Standard machines are 1016 mm χ 762
mm (40 in. χ 30 in.) giving a nominal gravity factor of 1800 at high speed.
Larger machines - 1220 mm χ 762 mm (48 in. χ 30 in.) - are also
available. The two-speed main drive motor is a nominal 900/1800 rpm
induction type. Because of the sticky character of the massecuite, the
discharge mechanism employs a "double-cut" system.
Spinning time varies depending upon the viscosity of the molasses and
the size and uniformity of the crystals. An average cycle time for
Hawaiian massecuites is 30 minutes. Heavy massecuites may require
double this. In regions with less viscous material, the cycle time may be
15 minutes or less.
Uniformity of crystal size is extremely important in obtaining good
molasses removal in batch machines. Even a small amount of fine crystals
will form an almost impervious layer within the sugar and will prevent the
molasses from reaching the screen. The molasses will then collect in a
layer, called a mirror, on the inside of the sugar wall and will run down
into the bottom of the basket when the machine is stopped. The same
condition can occur when finely divided extraneous matter is present. A
sometime cause is the accumulation of calcium magnesium aconitate in
the form of diamond-shaped crystals which, when they reach a size of
over 0.05 mm, can completely plug the sugar wall.
Machine curb top should be provided with a cover to reduce windage
and retain heat over long operating cycle of the machine.
115
CONTINUOUS MACHINES
Commercial Sugar
Continuous centrifugals have not proven to be satisfactory for
commercial sugar because of crystal breakage and difficulty of producing
a sugar of uniform high quality. It is possible to use continuous machines
on intermediate sugars which are remelted.
Low Grade Sugar
Continuous centrifugals have largely replaced batch machines in most
low grade installations. Continuous machines are much lower in first cost,
have higher capacity and lower power consumption. They have the major
disadvantage of giving a molasses purity increase across the machines.
This averages between 1 and 2% in most equipment, but unless carefully
controlled, can be substantially higher. Crystal breakage causes fines
which pass through the screen with the molasses. The crystal breakage
also renders the sugar unsuitable for the preparation of seed magma.
Continuous machines also require more operator attention than batch
machines as adjustments must be made in throughput rate and washing on
the basis of visual monitoring.
Continuous centrifugal baskets usually have a conical angle of about
30° from the vertical and are run at about 2200 rpm. Centrifugal screens
are usually chrome plated and have perforations of two types - circular
and slotted. Standard circular perforations are 0.125 mm in diameter.
Slotted perforations are usually 0.06 mm wide and up to 1.6 mm in length.
Although both types of perforations increase in size with wear, slotted
perforations are subject to deformation into elliptical shape resulting in
considerably greater enlargement.
Rate of massecuite feed is usually controlled by the load on the
V-belted drive motor. This is not a fully-automatic system as constant
load does not mean constant quality of product. However, it does give a
simple control and leaves quality in the hands of the operator.
The machines usually have distributors for application of water and
steam to the internal face of the cone. Both of these are difficult to keep
in optimum flow so without close attention the performance - that is
molasses removal without loss of sugar - varies widely. The most
satisfactory results are obtained by the use of steam alone introduced into
the massecuite feed pipe with none impinging on the screen. Optimum
performance is found with massecuites heated to the maximum
temperature permissible without solution of sugar.
Operation
Batch machines - Commercial sugar
Timing of the steps in the cycle must be adjusted to
characteristics of the massecuite which often vary with
Particularly important is the time of application and amount
mentioned before. Also, the final drying time must be set to
of the proper deterioration factor for storage.
the physical
each strike.
of water as
give a sugar
116
Batch machines - Low grade
Only two controls are possible - time of spinning and temperature of
massecuite. The time is often governed by that available to handle the
volume of massecuite, so the cycle is adjusted accordingly. The
massecuites should enter the machines at close to the saturation
temperature. The machines should always be closed to diminish air flow
and maintain temperature. Although steam is sometimes introduced
outside the basket, it is not advisable because of possible machinery
damage.
Continuous machines - Low grade
Constant attention is the key to good performance. The capacity of a
machine is that obtained with a given massecuite when operated to give
good quality sugar without excessive increase in molasses purity in the
machine.
Screens are very thin and so are subject to damage. Great care is
required in mounting the screens to insure that they fit tightly against the
backing so that no flexing occurs in operation. The massecuite should be
screened to remove lumps of sugar and extraneous material such as bolts
and nuts.
A target for maximum molasses purity increase in the machines of 1.0,
although rarely obtainable, is workable. Since the actual value is only
available to the operator at wide intervals from laboratory analysis,
observation of the sugar is the customary basis of control. With
reasonably good quality crystal, at the size level of 0.20 mm, the sugar
purity should be around 85. Decisions for adjustment of steam and
throughput are usually made on the basis of color. A better guide is
consistency. The sugar should be neither a dry powder nor have the
consistency of massecuite. It should have the consistency of a soft brown
sugar. An amusing but reliable measure is taste. If the sugar tastes sweet
it is overwashed (steamed). If it is bitter it is underwashed. A brackish
taste is about right.
117
Chapter 14
FINAL MOLASSES
The principal loss of sucrose in the boiling house is that retained in the
final molasses. This is not really a loss since molasses is a standard
commodity and is sold at a price which at times exceeds that of the value
of the sucrose content. The objective of the processor therefore is to
keep the sucrose content of the molasses at near the economic optimum
based upon the prevailing prices of sugar and molasses.
As sucrose crystallization from final molasses proceeds, the rate of
crystallization becomes slower and slower until finally a point is reached
at which no more sucrose will crystallize at a given temperature. The
molasses then is termed exhausted. The purity of exhausted molasses
depends principally on the water content. At the same water content,
however, the nature and quantity of the nonsucrose constituents, having
their origin in the cane, govern the purity. Although all constituents exert
an influence, only those present in large quantities have a major effect.
Of these, reducing sugars and ash are controlling.
These two ingredients have opposite effects upon sucrose retention.
Reducing sugars, in essence, take the place of sucrose almost weight for
weight, so the more reducing sugars present, the less sucrose will remain.
Therefore, the higher the percent reducing sugars, the lower the
exhausted molasses purity at a given water content. On the other hand,
ash in general, and potassium chloride (the main ash constituent) in
particular, tend to increase the solubility of sucrose giving a higher
exhausted molasses purity.
A statistically significant relationship exists between the reducing
sugars-ash ratio and the purity of molasses at saturation, i.e., molasses
from which no more sugar can be crystallized at a given temperature.
This relationship is the basis of the system established by the Experiment
Station of the Hawaiian Sugar Planters Association and used by factories
in Hawaii as a basis for judging sugar recovery potential from final
1
These are 50° C
molasses. Standard physical conditions are set.
temperature and 600 poises viscosity. A standard viscosity is essential
because of its controlling effect in the handling of massecuites, in the
crystallizers and in the centrifugal separation of sugar.
The following equations give the statistical relationship between the
reducing substances and ash composition and the expected purity of
molasses at 50° C and 600 poises. It is only necessary to analyze the
molasses for reducing sugar and ash and then insert the figures into the
equation to estimate the theoretical minimum purity - called "Expected
Purity" - that it is possible to obtain from the molasses. The difference
between this value and the purity actually obtained is called "Points
Above Expected."
118
Expected refractometer sucrose purity =
33.160 -
4.028(
reducin
gy
ances
)+
0.161 (refractometer sucrose purity)
or
33.253 - 5-351 ( ^ n S Î i t y Μ ? )
8
+
°'
1 34
<
r
e
f
r
a
c
t
o
m
e rt
e s
u
c
r
o es
P
u r i t
y>
In actual practice, Expected Purity, or the theoretical minimum purity
of exhausted molasses, will not be reached in a factory because complete
exhaustion may require as much as 30 days. For a given factory, how
close it approaches expected purity depends upon many factors. The most
important are the composition of the cane juice, the equipment such as
vacuum pans, crystallizers and centrifugals available, and the technology
used.
In general, also, the higher the ash content, the slower the
crystallization rate, so with a given capacity of equipment a factory
processing high ash juice cannot achieve as low a Points Above Expected
as one with low ash.
The lower the purity of the incoming juice, the more molasses will be
produced per unit sucrose and the greater quantity of low grade material
that must be processed. For example, 80 purity juice will theoretically
produce 2.4 times as much final molasses as 90 purity juice. So basically,
the capacity of the low-grade station should be at least two times greater
for the lower purity juice. With equipment limitations, high purity juice
should make it possible to reach lower purity molasses more readily.
Juice purities must therefore always be kept in mind when comparing
losses in molasses as the total sugar lost depends not only on the purity of
final molasses, but on its quantity.
The question, then, is how close to the expected purity should be the
target for factory operations. This depends upon the factors described
above and on the economics of the entire operation. The relative prices
of sugar and molasses must enter into the decision. As mentioned there
are times when the value of a unit of sucrose in molasses exceeds that in
sugar. This does not mean that it would pay to permit more sugar to go
into molasses. Molasses is sold by the ton, therefore sugar must be
accompanied by the normal amount of nonsucrose material to bring this
return. However, it does mean that the cost of recovering sugar beyond a
certain point may not be warranted.
Experience leads to the conclusion then, that a factory should have the
capability of processing final molasses to a level of 5 points above the
expected purity with incoming juices of average composition. Poor
quality juices will cause the molasses purity to increase because of the
effects on molasses quantity, rate of crystallization and viscosity factors over which the factory has no control.
Operational procedures being the same, the rate of crystallization of
sugar from final molasses is slowed down sharply by high ash content. The
119
component largely responsible is potassium chloride. Aside from ion
exchange treatment there is no effective means of removing this
compound. A solution would be to install more equipment which would
permit a longer crytallization time. However, in areas subject to a
normal condition of high ash, it is advisable to maintain a routine daily
measurement of the electrical conductivity of mixed juice so that the
operating staff will be able to predict the crystallization rate. It has been
found that there is a good correlation between the conductivity of mixed
juice and the points above expected of final molasses obtained from that
juice.
Another rate-retarding factor is elongated crystal growth. In cane
sugar processing this behavior is a characteristic of juices from
deteriorated cane. This is distinct from beet sugar processing where the
phenomonen is caused by raffinose. Small quantities of substances
produced in the growth of microorganisms in the cane, reportedly
oligosaccharides, accumulate preferentially on certain faces of the crystal
and inhibit the deposition of sucrose. Crystal growth then becomes
limited to the other faces and elongated crystals are formed. Often the
ratio of length to diameter is in the range of 3-5 to 1. In extreme cases
"needle grain" occurs in which the ratio is even higher. The rate of
growth of elongated crystals is much lower than that of normal crystals.
No practicable way of removing the interfering substances has been
found. When such crystals appear, the purity above expected of the
molasses increases markedly. A good indication to the operating staff of
problems ahead is the pH of first expressed juice. When this drops below
5.0 deteriorated cane is indicated and elongated crystals can be
anticipated.
The viscosity of final molasses governs the density to which the
massecuite can be boiled so that it can be cooled in the crystallizers and
handled mechanically. The higher the viscosity, the more the amount of
water that must be left in the molasses, so the purity will be higher.
Again the viscosity is related to the composition of the incoming juice so
the factory has no control. In general, however, high molasses viscosity is
found when juice from deteriorated, over-ripened, drought-stricken or
immature cane is processed. Advance knowledge of this incoming
material will be helpful to the operating staff in controlling the processing.
In conclusion, the expected purity values are useful operational
guides. In practice however, actual results must be appraised in light of
other factors also.
Other systems of estimation of expected purity are in use in other
regions. The Sugar Research Institute in Australia has developed a
formula based upon logic similar to that used in Hawaii. The formula is:
Expected true purity = 40.67 - 17.80 log
r e d u c
^jj[
s
u
?
a
r s
In addition to estimations based upon a fundamental approach,
numerous empirical formulae resulting from a statistical analysis of
actual factory results in given locales have been developed and used.
120
Chief among these are:
Douwes-Dekker (Java)
Expected purity = 35.886 - 0.08088 R + 0.26047 A
Where R = reducing sugars % nonsugars
A = sulfated ash % nonsugars
Hugot (Reunion)
Expected purity = 40 - 4 χ
r e d l
1
1
^ ^ g sugars
These are useful, practical guides under applicable conditions.
REFERENCES
1
Moritsugu, T., Somera, B. J., Sloane, G. E., Proc. Intern. Soc. Sugar
Cane Tech., 1974, pp. 1236-1245.
121
Chapter 15
RECOVERY FACTORS
The quantity of sucrose that it is possible to recover in sugar, relative
to the quantity present in the juice, is governed by the purity of the juice
and the purity of the final molasses. The relationship is shown by the SJM
formula:
Theoretical Recovery % = j | g _ ^ j
Where
S = Sugar purity
J = Juice purity
M = Final molasses purity
The formula is valid as long as the same units are used in expressing all
the purities. Standard is refractometer-pol purity.
The actual recovery will be less than the theoretical by the known loss
in filter cake and by unmeasured losses reported as Undetermined.
Included in the latter are mechanical losses and chemical decomposition
of sucrose in the processing.
Normal undetermined losses are
approximately 1.7%, made up of the following:
Clarification
Sugar Crystallization
Mechanical
0.2
1.0
0.5
Factory balance figures for a weekly or monthly period may vary ±
1.0% from the actual value because of the unreliability of estimations of
material in stock. No concern is needed, therefore, in the range of 0.5 to
2.5%. Figures greater than this should trigger checks of calculations,
measurements and sources of real losses.
The effect of juice purity is much greater than the effect of molasses
purity on recovery. This is shown in Table 15-1. A one point change in
juice purity from 85.0 changes the recovery 1% at 40 molasses purity,
whereas a change of one point in molasses purity changes recovery only
0.45%. A t lower juice purities, the figures are 1.1% and 0.66%,
respectively, showing relatively higher importance of molasses purity.
The reason for this is the rapid increase in the quantity of molasses with
lower juice purity. This is shown in Table 15-2, where molasses is
calculated as a percent of the pol in syrup. Molasses quantity increases
from 19.8% at 90 juice purity to 47.6% at 80 juice purity.
Thus,
purity is
molasses
recovery
it is important to keep in mind that obtaining low molasses
of far greater value with low juice purity than high. Reducing
purity from 40 to 25 at 80 juice purity gives an additional
of 8.5%, compared with only 3.5% at 90 juice purity.
122
TABLE 15-1
96 DA SUGAR RECOVERY % POL IN SYRUP*
y Molasses
\
Purity
40.0
35.0
30.0
25.0
93.0
101.5
102.3
103.1
103.7
92.0
100.6
101.7
102.5
103.3
91.0
99.8
101.0
102.0
102.9
90.0
98.9
100.3
101.4
102.4
89.0
98.0
99.6
100.9
102.0
88.0
97.1
98.8
100.3
101.6
87.0
96.2
98.1
99.7
101.1
86.0
95.2
97.3
99.1
100.6
85.0
94.3
96.5
98.5
100.1
84.0
93.3
95.7
97.8
99.6
83.0
92.2
94.9
97.2
99.1
82.0
91.2
94.0
96.5
98.6
81.0
90.1
93.2
95.8
98.1
80.0
89.0
92.3
95.1
97.5
79.0
87.9
91.4
94.4
97.0
78.0
86.7
90.5
93.6
96.4
Syrup
Purity
Refractometer pol purities.
refractometer solids molasses.
Assume
98.8
purity
sugar
and
84.0
123
TABLE 15-2
MOLASSES % POL IN SYRUP*
Molasses
Purity
40.0
35Λ)
3CL0
25.0
93.0
12.6
11.6
10.8
10.1
92.0
15.0
13.8
12.8
11.9
91.0
17.3
16.0
14.8
13.8
90.0
19.8
18.2
16.9
15.8
89.0
22.3
20.5
19.0
17.8
88.0
24.8
22.9
21.2
19.8
87.0
27.5
25.3
23.5
21.9
86.0
30.1
27.8
25.7
24.0
85.0
32.9
30.3
28.1
26.2
84.0
35.7
32.9
30.5
28.4
83.0
38.5
35.5
32.9
30.7
82.0
41.5
38.2
35.4
33.0
81.0
44.5
41.0
38.0
35.4
80.0
47.6
43.8
40.7
37.9
79.0
50.7
46.8
43.4
40.4
78.0
54.0
49.8
46.1
43.0
Syrup
Purity
\
Refractometer pol purities.
refractometer solids molasses.
Assume
98.8
purity
sugar
and
84.0
124
Table 15-3 shows the relationship between the quantities of sugar and
molasses at various purities.
TABLE 15-3
MOLASSES % 96 DA SUGAR*
\
\
Molasses
Purity
40.0
30
30
34.0
93.0
12.4
12.0
11.6
11.2
92.0
14.9
14.3
13.8
13.3
91.0
17.4
16.7
16.1
15.6
90.0
20.0
19.2
18.5
17.9
89.0
22.7
21.8
21.0
20.3
88.0
25.6
24.6
23.6
22.7
87.0
28.5
27.4
26.3
25.3
86.0
31.6
30.3
29.1
28.0
85.0
34.9
33.4
32.0
30.8
84.0
38.2
36.6
35.1
33.7
83.0
41.8
39.9
38.2
36.7
82.0
45.5
43.4
41.5
39.8
81.0
49.4
47.1
45.0
43.1
80.0
53.4
50.9
48.6
46.5
79.0
57.7
54.9
52.4
50.0
78.0
62.2
59.1
56.3
53.7
Syrup
Purity
*
Refractometer pol purities.
refractometer solids molasses.
Assume
98.8
purity
sugar
and
84.0
125
Since purities are ratios, the SJM formula is a ratio of ratios. When a
ratio changes either the numerator or the denominator or both may have
caused the change. For this reason, in order to obtain a true picture in
abnormal recovery situations, it is sound practice to run balances based on
weights of pol and weights of refractometer solids, comparing the
quantity in mixed juice with the total recovered in sugar and molasses.
Assuming that the accepted standard procedures in sugar crystallization are followed, the recovery actually obtained from a given
equipment installation is affected also by the quality of the juice
processed as well as its purity, as indicated in the chapter on Final
Molasses. Deleterious substances exert an influence not only on the
exhausted purity of the molasses, but also on the rate of crystallization
and the workability of the massecuites. Thus, the recovery obtained from
day to day may vary considerably as the quality of the juice varies in
other ways than purity.
The following are the principal adverse factors on recovery.
Low reducing substance-ash ratio.
Juice from deteriorated cane.
High content of alcohol precipitable matter.
The rate of sucrose crystallization decreases with the reducing
substance-ash ratio. This is caused usually by an increase in the salt
content - notably potassium chloride. In general, cane that is grown in
low-lying coastal areas gives juices of low ratios and ones that are
difficult to process. It is for this reason that there is a good correlation
between electrical conductivity of a juice and
its processing
characteristics. For example, a juice with a specific conductivity of 1000
micromhos processes much more easily than one of 3000 micromhos.
Juices with reducing substance-ash ratios above 1.50 are generally
easy to process and those below 1.0 difficult. When processing these low
ratio materials, there is no alternative to taking a longer time to boil the
pans and to allow more time in the crystallizers.
Juice from deteriorated (sour) cane will invariably result in elongated
crystals. Reported to be caused by some oligosaccharides produced by the
growth of souring microorganisms, the substances concentrate on specific
faces of the sucrose crystal inhibiting sucrose deposit, so only the
unaffected faces grow. A slow-growing crystal face tends to increase in
size. A fast-growing crystal face will decrease in size and may disappear
giving rise to such sucrose crystal forms as needles and triangles.
Since crystal growth in elongated crystals is limited on some faces, the
rate of crystallization is diminished, so a longer time is necessary to
obtain a desirable crystal yield. Also, the viscosity of the molasses from
sour cane is usually higher. The combination of these factors results in
difficult boiling massecuites, poor performance of crystallizers and
centrifugals, affecting both the processing rate and recovery. There is no
satisfactory solution to the problem, so normal practice is to process the
poor material at the standard rate, take a lower recovery, and get the
material out of the processing stream.
126
High content of alcohol-precipitable matter is normally associated
with sour cane but is also characteristic of drought-stricken cane. Syrup
and molasses from such cane has high viscosity so recovery is less because
the massecuites cannot be handled at high refractometer solids.
Again, there is limited action that can be taken, so recovery is lower.
It is better to operate pans and crystallizers at higher temperatures which
lower viscosity.
127
Chapter 16
SUGAR QUALITY
Commercial sugar consists of sucrose crystals with an adhering film of
molasses, so its quality depends on the composition and quantity of each
component. Established practice is to set standard specifications on the
quality of the crystal and on the sugar as a whole. Since the individual
crystal is relatively free of impurities, even though comprising most of
the sugar, the quality of the whole sugar is governed primarily by the
quantity of molasses. The higher the pol of the sugar, the less molasses
present, so the closer the analysis of the whole sugar approaches that of
the crystal.
Efforts to improve the quality of sugar take two courses - minimize
the amount of impurities within the crystal and reduce the quantity of
molasses outside the crystal. Material included within the crystal may be
kept to a minimum by use of boiling techniques which are directed toward
the growth of perfect crystals. Under the same conditions of growth, the
quantity and quality of the nonsucrose within the crystal will depend upon
the composition of the juice from which the mother liquor is derived. The
quantity of molasses retained on the crystal is governed by the operation
of the centrifugals and the physical character of the crystals.
The important raw sugar quality factors are:
Pol
Crystal size and uniformity
Color - crystal and whole sugar
Filterability
Moisture
Ash
POL
Since the crystal is close to 100% pure sucrose, the pol of the sugar is
determined by the amount of molasses surrounding the crystal. For
example, a sugar of 99.0 pol and 0.2% moisture would be composed of
approximately 1.3% molasses solids and 98.7% crystal solids, assuming the
molasses layer is exhausted. If the molasses layer is reduced further by
washing to a sugar of 99.5 pol, the quantity of molasses is approximately
halved. The limit that can be achieved in good raw sugar operations is
about 99.5 pol.
With uniform crystal of standard size (0.8 to 1.0 mm) and free of
conglomerates, the pol is controlled at the centrifugals by timing of the
spinning, wash and drying cycles and quantity of water used. With
irregular or conglomerate crystal, it is difficult to wash molasses from the
128
interstices without the use of excessive amounts of water which dissolve
sugar, lowering the yield and decreasing the crystal size.
CRYSTAL SIZE AND UNIFORMITY
Crystal size is controlled in the vacuum pans. The final size is
determined by the quantity of seed used relative to the final volume of
massecuite, making certain that additional nuclei do not develop during
the boiling. As discussed in an earlier chapter, this requires careful
control of the pan conditions and good circulation. Even under the best
controlled conditions, however, there is always the possibility of stray
nuclei forming and random crystals coming in.
Also, even under the same conditions, crystals of the same general size
do not grow at the same rate. This is because growth occurs most rapidly
at dislocations (irregularities) on the crystal surface, so the more
dislocations, the faster the crystal grows. The probability of dislocations
increases with the rate of growth also, so a self-perpetuating growth rate
system develops. For this reason, crystals grown under conditions of high
crystallization rate tend to be more uniform in size distribution, although
they will be less perfect in form. Since crystallization rate increases with
temperature, more uniform crystal distribution can be expected from
massecuites boiled at high temperatures.
Controlled washing in the centrifugals is necessary to avoid dissolving
sugar.
COLOR
Crystal
The color of the crystal from which the superficial layer of molasses
has been washed away is caused by absorption of colored components from
the mother liquor or entrapment of mother liquor within the crystal. If
conditions of crystallization are the same, then the color is determined by
the composition of the liquor. This means that the quality of the cane is
governing. Minimum color is present when clean, sound, mature cane
stalks are the raw material. Color increases with greenness of the cane,
deterioration of the cane and quantity of tops and leaf trash entering the
extraction plant.
Some color develops in the processing of the juice to sugar. In general,
processing conditions are relatively constant with respect to temperature,
pH and time, so this factor is not much of a variable. Little or no color
development occurs in evaporation. In sugar boiling, it is a function of the
product of time multiplied by temperature. Colored compounds are produced from the reaction between amino acids and reducing sugars,
decomposition of reducing sugars and many condensation type reactions.
But, as indicated, these take place under approximately the same conditions.
The operator has really only one way to keep color in the crystal to a
minimum, and that is to prevent inclusions in the crystal. The more
129
nearly perfect the crystals, the less the color. This means operating the
pans under controlled conditions so that the crystals grow uniformly
requiring, as a prime consideration, good circulation. In general, the
slower the rate of crystallization, the more perfect the crystals. Crystals
of sucrose that sparkle like diamonds can be produced under extremely
slow growth rates. Such are impractical on a commerical scale, of course,
so uniform fast rates are to be employed.
Formation of inclusions must be strictly avoided. These develop
especially when dissolution of the crystals is allowed to occur because of
conditions of undersaturation. As a crystal dissolves, which it does at
about five times the rate of crystallization, the corners and edges become
rounded first. Channels then develop on the surface. When such crystals
start to grow again, the edges of the channels, having more contact with
the supersaturated mother liquor, grow more rapidly and the channels
become overgrown, leaving mother liquor entrapped. Such crystals also
will likely have irregular surfaces and the entrapment phenomenon tends
to continue.
This reality points up the importance of having near perfect crystals
for seed. If imperfect crystals are used, or crystals that have been partly
dissolved, inclusion will start at the beginning and probably continue
throughout the growth of the crystal.
Also not to be overlooked is the common, but unacceptable, practice
of "washing out" false grain. This cannot be done without lowering the
quality of the remaining, partially dissolved crystals. If some dissolution
has taken place, the best procedure is to resume crystallization at a very
slow rate so that damage repairs can take place before rapid growth is
started. The alternative is to completely dissolve all crystals and start
over.
Since it is more difficult to maintain uniform crystallization conditions
over the surface of a large crystal than a small one, inclusions increase
with crystal size. For this reason, when grown under the same conditon,
the smaller the crystal, the less color. Optimum commerical crystals are
of uniform minimum size averaging 0.8 mm.
Whole Sugar
The color of the whole sugar is the sum of the color of the crystals and
the molasses - the major one being the quantity of the molasses. As
shown earlier for a 99.0 pol sugar, the composition is about 98.7% crystal
solids and 1.3% molasses solids. The average value of whole raw color is
about three times the crystal color, so two thirds of the color comes from
1.3% of the material.
Important factors in keeping the whole color minimal are uniformity of
crystal size, freedom from false grain and absence of conglomerates.
Cycle timing and washing efficiency in the centrifugals must be carefully
controlled. Two-stage washing is standard practice.
130
FILTERABILITY
Sugar filtration rate measurements are made on the washed crystals.
Therefore, quality considerations are the same as those for crystal color,
namely, that it is primarily the quantity and the type of material included
within the crystal that affects the filtration rate. It has been established
that the controlling element is the quantity of insoluble particles at the
size level of 1 micron. It has also been shown that the composition of the
particles is not important - only the size. Furthermore, almost any type
of material can be grown inside a sugar crystal. Likewise, the size can be
anything from micro to macro; for example, from colloids to such things
as grains of sand, string, or even rubber bands.
Although there is some absorption, which may be preferential, the bulk
of the filtration impeding insolubles enters the crystal by inclusion.
Therefore, the final condition is determined by the quantity of 1 micron
level insoluble material present in the mother liquor and the extent of
crystal inclusions.
Clarification which reduces the amount of micron material to a
minimum, of course, should be given first consideration. However, as
already noted, usually the effectiveness of clarification is more dependent
upon the quality of the juice rather than the clarification procedure. So
the most important factor that something can be done about is the sugar
boiling technique.
Good circulation, prevention of
inclusions
and
uniformity of crystal growth will lead to maximum filtration rates.
Again, other conditions being equal, the filterability is governed by the
quality of the incoming juice.
MOISTURE
There is water both on the surface of the sugar crystal and within the
crystal. That on the surface is related to the amount of the molasses
layer and the relative humidity of the atmosphere surrounding the sugar.
Moisture within the crystal, again, is governed by the extent of inclusions
of mother liquor.
As moisture is of primary importance in maintaining the keeping
quality of the sugar relative to the growth of microorganisms, it is the
superficial moisture that is important. This can be controlled in the
operation of the centrifugals with proper maintenance of the washing and
drying cycles.
The Deterioration Factor or Safety Factor is used as an indicator of
the keeping quality of sugar with respect to moisture:
Deterioration Factor =
f^
1Stur
?
100 - pol
This is generally applicable to sugars in the pol range of 97.5 to 98.5,
where a factor of 0.25 is considered safe. For high pol sugars, 99.0 and
above, 0.20 is a safer limit, although moisture is less of a factor for such
sugars.
131
With all sugars, safe moisture levels can be obtained in the centrifugal
without the use of sugar dryers as is common practice in some sugar
regions. Conditioning of the sugar, that is allowing it to come to
equilibrium with respect to internal moisture, external moisture and the
atmosphere, is an important step in safe storage and prevention of
caking. Conditioning can well be accomplished in a "dryer" operated
essentially as a cooler.
ASH
Ash is no longer a factor of much importance in a raw sugar because of
higher pols. It was traditionally run on the whole sugar and was really a
measure of the quantity of molasses present. Ash level of the sugar
crystal is a function of the amount of inclusions, as in the case of color
and filterability.
133
Chapter 17
SUGAR AND MOLASSES HANDLING
SUGAR
The important factors in the storage of bulk raw sugar are the
pol-moisture relationship, the temperature of the sugar, and the relative
humidity of the air in the storage enclosure. The initial pol-moisture
relationship determines the susceptibility to action by microorganisims.
Temperature governs the rate of chemical decomposition (particularly of
the molasses layer) and caking tendency. The relative humidity affects
the maintenance of the initial moisture and the caking or liquifying trend.
In the general pol range of 97.8 to 99.3, growth of microorganisms is
inhibited if the factor,
Moisture
100 - pol
is not much above 0.25. This ratio is called the Deterioration Factor or
Safety Factor. Principally involved is the moisture level of the molasses
layer surrounding the crystals. If the sugar were not washed in the
centrifugal, then the keeping quality of the sugar would not be impaired
for microorganisms will not grow in a saturated molasses because of the
high osmotic pressure. Washing dilutes the molasses and permits growth.
Growth of microorganisms apparently causes further dilution and loss of
sugar becomes continuous, although slow.
First involved in the
microorganism activity are reducing sugars, and in particular the
fructose. Thus, in the initial stage, the pol of a relatively low pol sugar
will actually increase, because of fructose loss, before the pol starts to
decrease as sucrose inverts. Care in the washing and drying steps in the
centrifugal will give a sugar with a Deterioration Factor in the safe range
without much difficulty.
The molasses layer has a definite vapor pressure (water vapor)
depending upon the temperature and to some extent on composition.
There is for every sugar a humidity at which it neither gives up nor
absorbs moisture. This is called the Equilibrium Relative Humidity. For
raw sugars in the safe range this is about 65% at ambient temperatures.
If the atmosphere in which the sugar is stored contains less moisture, then
water will evaporate, the molasses concentrate and the sugar become
drier. If it contains more moisture, water will be absorbed and the sugar
becomes wetter.
Sugars stored at a relative humidity much below this will tend to cake
because of sucrose crystallization caused by molasses concentration and
cementing together of crystals. Sugars stored at a relative humidity much
134
above can reach the dilution level where microorganism activity increases. A t very high humidity the sugar will completely liquify. The
point at which this occurs is somewhere above 85% relative humidity.
Storage of raw sugar, however, does not require a controlled humidity
of 65%. In a relatively closed warehouse only the sugar in the surface
area of a large bulk pile is usually subject to the atmospheric effects
found in most subtropical and tropical regions. There is a general diurnal
pattern in which, during the middle of the day, the humidity above the
storage pile drops below 65% and the surface crusts over. Late at night,
when the humidity may go above 95%, the surface liquifies. This drying
and liquifying action continues only at the surface, seldom extending
below 25mm. In cases where rain continues for several days keeping the
humidity high in the daytime the surface may remain wet. It is important
in such circumstances to keep the storage area completely closed in order
to prevent entrance of much outside air.
Bulk storage is much better than bag storage as the action described is
not possible with bags. In bags a large surface area is exposed and
therefore a substantial quantity of sugar is involved in the humidityproduced action. Furthermore, if the bag is jute or cotton, the material
acts as a wick transporting the liquified sugar to the outside of the bag,
removing it from the sugar surface and ultimately permitting it to flow
away as molasses. Sugar loss can become high in areas of constantly high
humidity unless completely closed storage is used.
Temperature is the second important factor in storage. The molasses
layer on the crystals is subject to the same danger of exothermic
decomposition as final molasses if the temperature of storage is elevated.
The critical temperature for sugar is about the same as molasses - 45° C at which the chemical reaction responsible becomes self-sustaining. When
insulated by a large volume of material, as in the center of a pile of sugar,
the heat builds up to a point where complete decomposition can occur.
This has happened with sugar as well as molasses. Sugar is more
susceptible the lower the pol - that is, the more molasses it contains.
High pol sugar (99.3 or above) is in little danger. Even if the temperature
of the sugar does not reach the critical point, serious darkening of the
sugar can occur. Although the action is mostly in the molasses layer, it
may extend to the crystal also. Sugars subject to this thermal action have
a reddish color. Raw sugar therefore should not enter a bulk storage
facility above 40° C.
Caking of stored sugar, particularly in bags, can also be caused by
storage of hot sugar. On cooling, sugar in the molasses layer crystallizes
causing a cementing together of the crystals. Sugars which have been
conditioned by cooling while being agitated, either by air or mechanical
means, are less subject to caking. This is a standard practice with refined
sugar.
MOLASSES
The standard for final molasses for shipping is 85 brix at 1-to-l
dilution. This corresponds to approximately 82 refractometer solids.
135
Final molasses at this concentration is not subject to microorganism
activity except superficially, because of its high density, but is thermal
sensitive. As noted in the discussion under sugar, complete decomposition
can occur if the molasses temperature reaches a threshold value. This is
approximately 45° C. Evidence indicates that the triggering chemical
reaction is that between reducing sugars and amino acids. This reaction is
exothermic and carbon dioxide is released. A t low temperatures, the
reaction proceeds slowly with only a small evolution of carbon dioxide. As
the temperature increases, evolution of gas is sufficient to cause the
molasses to increase in volume and frothing occurs. In the absence of
circulation within a large body of molasses, the heat is not dissipated and
the reaction can become rapid and violent with complete decomposition,
leaving only a char-like residue.
Preventive measures are based on temperature control. Molasses, like
sugar, should be cooled below 40° C before entering storage. The storage
tank should be fitted with circulation or agitation equipment to give
sufficient mixing to prevent hot spots from developing. A satisfactory
procedure is the use of compressed air piped to the bottom of each of the
quadrants of the tank. When frothing begins, air will provide both cooling
and circulation. Storage tanks should be fitted with temperatureaccuated systems which automatically start the air pumps at a set
temperature.
Molasses at 25° C changes very little in composition over a period of
years. In storage there may be slight microbial activity on the surface
made possible by localized dilution from moisture condensation. Such
activity is minor unless the humidity is high. The surface of stored
molasses must be protected from casual water.
137
Chapter 18
STEAM GENERATION
A cane sugar factory is captive energywise, obtaining its power and
heat requirements by burning its own fuel - bagasse. A factory designed
for energy efficiency and properly operated will produce surplus bagasse
from which electricity can be generated and exported.
BOILER DESIGN
The design of a bagasse boiler used primarily to supply factory power,
process steam needs and plantation power load (such as irrigation) tends to
center on one capable of burning bagasse at 48% moisture and producing
2
2
steam at a nominal 32 k g / c m (450 lb/in. ) (3103 kPa) pressure. If export
2
of more power is favorable, then doubling the pressure to 64 k g / c m (900
2
lb/in. ) (6206 kPa) will be justified. The cost of boilers above the 32
kg/cm* range increases substantially, however. Also operation control
becomes more critical. In particular, boiler water quality becomes of
great importance.
With finely prepared bagasse, spreader stoker feed permits burning
most of the fuel in suspension. Travelling grates allow burning of the
remainder on the grate and give effective removal of ash. Suspension
burning also gives a faster response to load changes.
High boiler efficiency requires keeping the losses to a minimum.
These tend to increase as the boiler pressure increases. The use of air
heaters and economizers constitutes the principal means of reducing
sensible heat loss. These are heat exchangers in the flow of the exiting
gases from the boiler, which transfer heat to the entering air in the case
of air heaters and to boiler feed water in the case of economizers. The
industry still awaits the development of a system for recovering the latent
heat of the water vapor in the flue gases. Effective insulation is essential
to reduce losses from the boiler itself.
Design specifications must include stack emission control equipment.
The most efficient of these are wet scrubbers. Because of corrosion,
these must be constructed of corrosion-resistant material. Gas velocity
must be kept as low as practicable in order to increase the efficiency of
the scrubbers.
CONTROL
Boiler Efficiency
The overall efficiency of a boiler is expressed as the percentage ratio
between the heat transferred to the steam and the heat available in the
fuel. The formula is:
138
Fff *
<* - Heat transferred to steam χ 100
fcinciency <*> - Gross Calorific Value of bagasse
A value of 68% is considered to be a reasonable target for an efficient
bagasse boiler. Actual measurements of efficiency are rarely conducted
because of the difficulty of measuring the weight of bagasse entering the
boiler in a given time period. Useful approximations of the efficiency are
possible however.
A boiler heat balance is composed of the sum of the following items:
Transferred to steam (Efficiency)
Condensation loss
Sensible heat loss
Unburnt gas loss
Blowdown loss
Undetermined
Condensation loss is the latent heat of water contained in the bagasse.
Sensible heat
temperature.
loss
is
sensible
heat
of
flue
gas
above
ambient
Unburnt gas loss is due to incomplete combustion to carbon monoxide
instead of carbon dioxide.
Blowdown loss is due to the necessary continuous drain of boiler water
to maintain dissolved solids in the boiler to safe operating level.
Undetermined includes loss to surroundings, unburnt solids and furnace
ash.
Analysis of the flue gas, temperature measurements and visual
observation, together with an estimate of undetermined losses give a good
estimate of boiler performance.
Gross Calorific Value of bagasse is the total heat of combustion, which
for dry, ash-free bagasse fiber is taken as 4643 cal/kg (8350 Btu/lb) (19422
kJ/kg). For bagasse as fired, correction must be made for moisture, ash
and soluble combustibles. Assume a typical bagasse of 48% moisture, 2%
pol and 2% ash, then
Fiber % = 100 - [48 + 2(+) + 2]
The soluble combustibles would be somewhat higher than the pol.
However, the calorific value of sugar, 3953 cal/kg (7110 Btu/lb) (16538
kJ/kg), is less than that of fiber, so it is close enough to consider pol as
having the same fuel value as fiber. Therefore,
Fiber % = 100 - 52 = 48%
Gross Calorific Value
= 0.48 χ 4643 = 2229 cal/kg
0.48 χ 8350 = 4008 Btu/lb
0.48 χ 19422= 9323 kJ/kg
Net Calorific Value is less than the gross by the amount of heat
necessary to vaporize the water in the original bagasse and that from
139
combustion (condensation loss). A standard deduction of 573 cal/kg (1030
Btu/lb) (2396 kJ/kg) of water formed is often used.
Condensation loss as a percent of Gross Calorific
1
estimated by the formula:
~
,
4.· ι
100 (562 - 4.82 χ moisture % bagasse)
Condensation loss =
Gross Calorific Value (Btu)
Value can be
In the above example this is:
100 (562 - 4.82 χ 48) _ 0 O
8 2
4008 (Btu)
·
Q
L
%
Sensible heat loss is determined by the temperature and composition of
the gases leaving the boiler. This loss as a percent of the Gross Calorific
1
Value can be estimated by the Siegert formula:
K
Sensible heat loss % = Ç Q
+\j
The constant Κ depends upon the C O 2 content of the flue gas and the
moisture content of the bagasse, tp and t^ are flue gas and ambient air
temperatures. C O 2 is volumetric % of carbon dioxide and U is volumetric
% of unburned gas converted to carbon dioxide.
This formula can be simplified to:
Sensible heat loss = Κ (tp - t ^ ) without unburned gas
The following
temperatures.
table
shows
Κ
values
relative
to
VALUES OF 1,000 Κ FOR SENSIBLE HEAT LOSS
Fahrenheit
1
Moisture in Bagasse %
in
gas %
CO2
6
8
10
11
12
13
14
15
16
17
\
44
46
48
50
52
68.1
53.4
44.5
41.3
38.6
36.4
34.4
32.7
31.3
30.0
68.4
53.7
44.9
41.7
39.1
36.9
34.9
33.2
31.7
30.5
68.8
54.1
45.3
42.1
39.4
37.2
35.2
33.6
32.1
30.8
69.4
54.6
45.8
42.6
39.9
37.6
35.7
34.1
32.6
31.3
69.9
55.1
46.3
43.1
40.4
38.2
36.2
34.5
33.1
31.7
140
In the example with 48% bagasse moisture, 14% C O 2 and
temperature difference of 300°F, the sensible heat loss would be 10.6%.
a
Unburnt gas loss can be measured by analysis of the carbon monoxide
1
content. As a percent of Gross Calorific Value the formula is:
Unburnt gas loss % = 42
where U is volume % of unburnt gas converted to C O 2 .
Undetermined loss consisting of radiation loss, loss of sensible heat in
ash and loss in unburned bagasse particles can vary widely but is usually in
the range of 5 to 10%. An average value is estimated for making
efficiency calculations.
Summarizing the estimations based upon the example previously cited,
the total would be:
% of gross
calorific value
Transferred to steam (Efficiency)
Condensation loss
Sensible heat loss
Unburnt gas loss
Blowdown loss
Undetermined
Total
68.0
8.2
10.6
4.5
3.0
5.7
100.0
BOILER WATER
Feedwater for steam generation should not cause scaling or corrosion
in the boiler and should give steam free from contaminants. The best
source of water meeting these requirements is from condensation of the
steam itself. Thus, the condensate of exhaust steam is the principal
source of supply water. As the quantity is not sufficient because of
losses, some makeup is necessary. This is often taken from the following
effects of the evaporator, juice heaters and pans when these are heated by
vapor from the evaporator. Treated raw water is used only when the
supply of condensates is insufficient.
Exhaust steam condensate should be essentially distilled water subject
to contamination only by carryover from the boiler and leakage in the
heat transfer vessels where the exhaust steam is used. The exhaust from
reciprocating steam engines will also contain oil. Leakage of sugarbearing material should not occur in evaporator cells and pans heated by
exhaust steam since the calandria pressure is higher than the body
pressure. Of course when the vessels are shut down there is a possibility
of contamination. In juice heaters, however, the juice is at higher
pressure and a leak can cause serious leakage into the condensate.
Condensates from vapors are called vegetal condensates and contain
volatile organic compounds from the cane juice, chief of which is ethanol.
141
Also present are organic acids, higher alcohols, esters and oils. Although
such materials are not a problem in low-pressure boilers, they are of
2
2
concern with pressures above 32 k g / c m (450 lb/in. ) (3103 kPa) because
they can cause priming and carryover. Treated fresh water should be used
for make up in these boilers.
Raw water must be treated chemically depending upon its quality and
the boiler pressure.
This includes removal of insoluble solids,
precipitation of scale-forming constitutents, increasing pH, and in some
cases removal of soluble constituents by ion exchange.
Boiler Water Treatment
Standard boiler water treatment requires:
1.
Deaeration to remove oxygen by flashing
2.
Heating
3.
Chemical treatment
4.
a.
to inhibit scaling
b.
to prevent corrosion
c.
to reduce foaming or carryover
Blowdown to reduce accumulation of solids
Deaeration
Much of the dissolved oxygen in feedwater can be removed by
deaeration. This is accomplished by heating and allowing the feed to flash
to lower pressure.
Heating
All condensates should be maintained without cooling. With an
economizer the feedwater is heated by flue gas before entry to the boiler.
Chemical treatment
To inhibit scaling, the principal elements to immobilize are calcium,
magnesium and silica. Normal treatment consists of making alkaline with
caustic soda and adding phosphate. This causes precipitation of calcium
phosphate, magnesium hydroxide and some silicates. These insoluble
materials are removed from the boiler as a sludge in the blowdown.
Chelating agents, which sequester positive ions, are part of some
treatments. They are effective in removing copper and iron.
An increase in alkalinity prevents acidic corrosion. To prevent
oxygen corrosion, oxygen scavengers are used. Those commonly used are
sodium sulfite and hydrazine. Hydrazine is preferable in high pressure
boilers as it does not increase the solids content.
Priming and foaming, which cause carryover of the boiler water into
the steam, are usually caused by the presence of surface-active substances in the water. Although antifoaming agents are often used to
reduce priming, there is no effective treatment for high pressure boilers,
so the water should be free of any organic material which might cause
142
priming. Priming is more serious when the load oscillates, so a steady
steam flow is always desirable.
Blowdown
Blowdown prevents the build up of insoluble solids. Since blowdown is
wasteful of energy it is important to keep solids in the water to a
minimum. This also means keeping water treatment chemicals to a
minimum.
OPERATION
A steady uniform operation is vital to maintain efficiency in a
boiler. This means a constant supply of bagasse of uniform quality. If
milling stops, stored bagasse must be introduced without interruption.
Bagasse
Bagasse quality is of prime importance and moisture content is the
most important factor. Most boilers are designed to burn bagasse of 50%
moisture. Problems in burning can be expected if the moisture goes above
52%. Much of the bagasse does not dry and burn in suspension but
accumulates on the grate. In the case of a travelling grate, there may not
be much difficulty but on a dumping grate, bagasse may accumulate in
piles, combustible gases can be generated which ignite explosively from
time to time causing puffing.
Bagasse with green leaves dries slowly, even though the
content is satisfactory, and will cause burning difficulties.
moisture
The steam generating capacity of the bagasse is a function of the
moisture content. As a rule of thumb, a one percent change in moisture
means about a one percent change in the fuel value obtained fom a given
quantity of fiber entering the mill.
Stored bagasse dries out and becomes a more efficient fuel. However
it also loses its sugar content rapidly which means a loss in energy. With
good mill extraction this amounts to the order of 3% of the total energy
available. Thus, in short periods of storage (2 or 3 days) the two effects
are somewhat balancing. Bagasse of high sugar content should be burned
at once. It should be kept in mind in this connection that with poor mill
extraction bagasse may show a low moisture content simply because the
pol is high.
Air Supply
Air supply must be adjusted to the bagasse rate using the minimum
amount of air necessary for combustion. In practice it is not possible to
get complete combustion without using excessive amounts of air, so a
reasonable balance is arrived at leaving some unburnt gases.
In the absence of analysis of flue gas for unburnt gas content, control
can be by C O 2 or oxygen content. In reality, routine control is normally
based upon air pressure measurements in the boiler.
143
Boiler Water
Standard boiler water control procedures are used. The basic
requirements of which are to maintain alkalinity, a scale treatment
additive - phosphate or chelate, a residual oxygen scavenger content and a
low level of solids.
Condensates are monitored continuously for contamination caused by
equipment leaks. Electrical conductivity is commonly used for sensing as
the sugar streams carry relatively high salt concentrations. Contaminated
condensate should be discarded to the house hot water system.
Oil Firing
Oil firing is used to supplement bagasse as required. It is, of course,
vital to keep its usage at a minimum. This requires careful management
of the bagasse supply and steam demands of the factory. Since fuel oil
has some five times the calorific content of bagasse it is difficult to burn
the two fuels together with good efficiency.
Stack Emission
Government regulations require careful control of the particulate
matter in the stack emission. Even with effective wet scrubbers control
of particulate matter is difficult with variable loads, so it is important to
maintain steady operation. Bagasse of high moisture content or
containing green trash does not burn readily and produces more unburned
materials. Much of the fine particulate matter also has its origin in soil
present in the bagasse.
BAGASSE DRYING
The fuel value of bagasse becomes higher as the moisture content
decreases mainly because of less heat necessary to vaporize the water.
The most energy-efficient means of removing water is to do it
mechanically in a press or a mill, but there is a practical lower limit. The
traditional standard for a cane mill is to get a bagasse of 48% moisture.
Mills have operated in the past with yearly averages below 40%, however.
This would be considered uneconomical now because of higher milling
rates. About the lowest considered practicable is 45% average moisture.
There is sensible heat in flue gas which could be used to evaporate
some of the moisture from bagasse. A bagasse dryer utilizing flue gas is
essentially a heat trap like an air heater or economizer. If the stack gas
has a high enough temperature to evaporate some water and still keep
well above the dew point then a bagasse dryer heat trap may have
economic value.
In a boiler with a high stack temperature of 250° C a bagasse dryer
working on 46% moisture bagasse which halved the temperature to 125°C
would increase the calculated boiler efficiency by approximately 7.8%.
From this must be subtracted the energy used in the drying system such as
for driving fans and also radiation losses. In an efficient boiler with air
heater and economizer the temperature of the stack gas is already close
144
to 125°C so the sensible heat available above the dew point may be too
low for recovery by bagasse drying. It must be kept in mind that all of the
water vapor originally in the bagasse remains in the stack gas after a
bagasse dryer.
FIBROUS TRASH FUEL
There is a quantity of potential fuel that can be made available to the
factory from fibrous cane trash - that is, tops and leaves. During its
growth a 24 months cane plant produces close to twice as much fiber in
leaves as is present in the millable stalk at harvest. Most of the leaves
drop off and become ground trash before harvest. Some of these
decompose but essentially all, as well as attached dry leaves, is burned in
a dry weather cane fire. There is left only a small partially dried tops
which averages in weight only about 3% of the total cane harvested, and a
bit of unburned leaves amounting to about 1/2%. The tops average around
25% fiber and the leaves from 35% to 70% depending on their dryness.
Using these figures there is available from completely burned cane per
100 tons stalk cane of 13% fiber:
Tons fiber
Stalks
Tops
Leaves (50% moisture)
13.00
0.78
0.26
14.04
This is 8% more than in the stalks alone.
Under the same conditions if the cane is not burned the weight of tops
would be about 4% but dry leaves would increase to about 11%. The
figures per 100 tons stalk cane would then be:
Tons fiber
Stalks
Tops
Leaves (50% moisture)
13.00
1.18
6.47
20.65
This is an increase in fiber of 59% over that in the stalks alone.
The relatively small amount of fiber from trash in well-burned cane
can be processed in a mill or diffuser without much difficulty, provided
there is a capacity for the increased load. Pol in bagasse can be held at
approximately the same level as without the trash. Extraction, that is
recovery, will be lowered by the amount of additional bagasse. A t the
95% extraction level this would correspond to a drop of about 1/2% in
extraction.
Processing of unburned cane, aside from harvesting and transportation,
would necessitate roughly a 50% increase in extraction plant capacity to
achieve the same pol in bagasse. Assuming that this capacity were
available the extraction would decrease by about 3%. Under such
145
conditions handling of the trash separate from the cane would have to be
considered.
Two options are available for handling the fibrous trash from the
cleaning plant. In one, the trash may be shredded and partially de watered
in a mill then returned to the bagasse. The moisture content of the trash
would be around 62%, so the moisture of the bagasse would have to be low
in order to provide a boiler feed of bagasse and trash of 50% moisture. In
another, the trash is trucked to an open storage area where it is allowed
to air-dry. It can then be returned to the bagasse supply for feed to the
boiler.
REFERENCES
1
Laboratory Manual for Queensland Sugar Mills 5th Ed. Bureau of
Experiment Stations, Brisbane 1970, pp 169-170.
147
Chapter 19
USE OF STEAM
The uses of high pressure steam for power and exhaust steam for
processing make a cane sugar factory uniquely efficient in energy
utilization. However, there is always difficulty in establishing a balance
between the two demands. This arises to a large degree, from the
variable process demand of batch vacuum pans. Thus, if sufficient
exhaust steam is generated to meet peak demand (when all pans are on
stream), then when some pans are down exhaust steam may be lost to
atmosphere, or vapor lost to atmosphere (if the pans are operated on
vapor.) This gives not only a loss in energy but also a loss in condensate
for boiler feed water. To reduce this, therefore, it is common practice to
generate less than the full demand of exhaust steam and make up the
remainder by passing high pressure steam through a pressure reducing
valve.
To determine just how much exhaust steam should be generated
requires a careful study, not only of the steam requirements of the several
stations, but also of the general experience of how the factory runs with
respect to cane supply, juice quality, extraction plant time schedule and
time efficiency. A general rule of thumb is that about 80% of the peak
processing demand should be supplied as exhaust steam. This figure should
be used only as a starting point and not as a valid figure for an individual
factory.
STEAM PRESSURES
2
As discussed earlier, the optimum boiler pressure is around 32 k g / c m ,
2
(450 lb/in. ) (3103 kPa). This makes possible the meeting of the power
demands of the factory, outside requirements for irrigation pumps and
still leaves some available for export of electricity. Of course, with a
profitable export electricity market, it is advisable to use the greater
2
power generating efficiency of higher pressure. In such cases 64 k g / c m
2
(900 lb/in. ) (6206 kPa) is reasonable.
Exhaust pressures should be the minimum necessary to meet the
processing uses because the higher the back pressure on the prime movers,
the less their efficiency. The minimum is actually determined by the
requirement for heating limed juice which must be flashed as it enters the
clarifier. The minimum heating temperature is therefore 102° C. The
first effect evaporator vapor used for heating therefore has to be about
108°C in order to give adequate heat transfer. This means a pressure of
2
2
0.35 k g / c m (5 lb/in. ) (34 kPa). In order to obtain this pressure, the
exhaust steam should enter the first effect of the evaporator at a
2
2
minimum of 0.75 k g / c m and preferably up to 1.0 k g / c m . The back
148
pressure at the prime movers should therefore be in the range of 1.0 to 1.4
2
2
kg/cm (14-20 lb/in. ) (97-136 kPa).
STEAM BALANCE
Traditional steam balances for a factory usually involve a series of
calculations based on assumptions and approximations. Rarely are actual
heat and power requirements measured and individual steam flows
recorded. Calculated balances are useful exercises and give the order of
magnitude of the requirements, but are less useful in appraising a going
operation in order to achieve better efficiency. Actual measurement of
steam being used is the best lead to improved economy.
The factory loads outside the boiler are as follows:
High pressure steam
Turbogenerators
Prime movers
Makeup
Exhaust steam
Evaporator
Juice heaters
Vacuum pans
Miscellaneous (centrifugals,
molasses heating, steaming out,
crystallizers, hot water)
The total quantity of high pressure steam depends upon the power
requirements of the factory equipment. With most of the prime movers
electrical, except for large units such as for the mill or diffuser stations,
the turbogenerator will be the main consumer in providing electricity for
factory loads as well as export. Power necessary for the extracting plant
is variable depending largely on how much is used in the cane preparation
machines. With a given installation there is not much that can be done in
operation to improve efficiency.
In the case of exhaust steam, however, efficient operation is most
important. Here most of the steam goes to the evaporator, with pans and
juice heaters operating on vapor. Flows to these stations should be
monitored and recorded. Recording is particularly important in pointing
out the load fluctuations. Once the amount of steam being used at each
station is known, steps can be taken to make more effective use of the
available steam.
PROCESS STEAM REQUIREMENTS
Evaporation
In a quadruple
1
follows:
effect
evaporator typical conditions might be as
149
Pressure
kg/cm^ lb/in.2
Exhaust
1st effect
vapor
2nd effect
vapor
3rd effect
vapor
4th effect
vapor
Vacuum
mm Hg in. Hg
0.91
13
—
—
0.49
7
—
0
0
—
—
Temperature
»_c
Latent heat
cal/kg Btu/lb J/g
119
246
527
948
2205
—
111
232
532
957
2226
0
0
100
212
539
970
2256
—
381
15
82
179
550
990
2302
—
648
25.5
55
131
568
1022
2377
If no vapor is taken for juice heaters and vacuum pans, one unit of
steam will evaporate approximately one unit of water in each individual
effect. It will be noted from the table that there is almost enough latent
heat from condensation to provide the latent heat of the vapor in each
case. There is however some flash evaporation as the juice passes from a
higher pressure to a lower in each effect. If this flash is utilized it will
more than compensate for the latent heat necessary. In actual practice
nevertheless, the steam required is somewhat more than the one-to-one
relationship because of losses in operation of the multiple effect system.
For approximation purposes the one-to-one ratio may be used.
Assuming 100 units of clarified juice of 13% solids to be evaporated to
65% solids, the quantity of water vaporized will be:
100 -(100 x 0 . 1 3 ) ,
OS
Qn
8 0
Since each effect will evaporate the same quantity of water the first
body will need:
80
- r = 20 units of exhaust steam.
4
Juice Heating
The rule of thumb for juice heating is that one unit of steam will heat
100 units of juice 10°F. This decimal relationship in English units comes
about from the fact that the specific heat of juice is 0.96 and one pound
of steam has roughly 960 Btu latent heat.
For 100 units of clarified juice there would be about 90 units of limed
juice to heat to the flash point, say from 85° F to 218°F, or 13 3° F total
133
-j-g- χ 0.9 = 12.0 units steam would be necessary.
In addition, the clarified juice will have to be heated from about 202°F
to its boiling point in the first effect, 232° F, or 30° F total regardless of
whether this is done in a preheater or in the evaporator. For 100 units of
juice it will take:
30
• j ^ χ 1.0 = 3.0 units of steam.
150
Sugar Boiling
Calculations of the amount of steam necessary for sugar boiling are of
limited practical value because the calculated amount is greatly exceeded
by addition of water during the boiling. Using average values of solids
content for the various massecuites, molasses and limited inboiling, the
calculated steam necessary to boil massecuites from 100 units of clarified
juice is about 10 units. In practice the actual amount ranges from 11 to
16 units. So a reasonable assigned value might be 14 units.
Total Process Steam Required
With no vapor bleeding from the evaporator the total steam per 100
units clarified juice would be:
Units steam
Evaporation
Juice heating
Limed juice
Clarified juice
Sugar boiling
20
12
3
U
49
With vapor bleeding, if first vapor is used for heating limed juice and
sugar boiling, then
12 + 14 = 26 units of vapor
are required. Thus 80 - 26, or 54 units will have to be used for evaporation in the quadruple, or
= 13.5 units
4
in each effect.
The total units of steam required entering the evaporator, therefore
would be 26 + 13.5 = 39.5. Add to this the 3 for clarified juice heating
gives a total of 42.5, compared with 49 without bleeding. To these figures
should be added about 10% to cover miscellaneous losses.
POWER RELATIONS OF STEAM
The power available from steam in a turbine is dependent upon the
pressure and superheat of the steam and the back pressure of the exhaust.
1
These are illustrated in the following table from Eisner :
151
Heat
Steam Steam
Back
availHeat utilized
pressure temp, pressure able 50% Eff. 65% Eff.
lb/in.a Btu/lb Btu/lb
lb/in>
F
Btu/lb
Steam required
50% Eff. 65% Eff.
lb/hp/hr
lb/hp/hr
180
180
180
380(sat.)
500
500
15
15
10
140
160
172
70
80
86
91
104
112
36.5
31.5
29.5
28.0
24.5
22.7
250
250
250
407(sat.)
500
500
15
15
10
166
180
196
83
90
98
108
117
127
30.5
28.0
26.0
23.5
22.0
20.0
OPERATION
Steady operation is the critical factor in maintaining steam usage
efficiency.
A reliable cane supply must be available so that the
processing rate, once set, can be held with minimum fluctuation. A
stop-and-go situation means excess and shortage of steam.
Also for every factory there is a minimum cane rate at which the
bagasse produced is sufficient to supply the fuel needs. This minimum
should be established and the factory should not attempt to run below that
rate except on an emergency basis.
If the design steam balance of the factory is such that 80% of the
process steam is supplied by exhaust steam, then at full load the makeup
valve will be open and no steam is lost. Should the processing load drop
below the 80% level because pans are shut down, then exhaust steam will
have to be wasted by blowing to atmosphere. This can be prevented
visually in large factories with several pans but is difficult in a small
factory with a few pans. Steady pan operation is important therefore, a
condition which is helped by having sufficient storage capacity for syrup
and intermediate molasses.
When the factory is operating in the range where makeup is being used,
it is vital to keep the makeup to a minimum. This means good control of
the quantity of water entering the process streams. First action in
reducing water usage is usually to cut down on imbibition water to the
mills or diffuser. Unless the imbibition is above the optimum, however,
extraction will suffer. It is prudent to fix the imbibition rate at the
optimum and not sacrifice in extraction. Often water use around a milling
tandem can be lowered. Large quantities of water can get into the mixed
juice from washing down the juice pans and allowing bearing cooling water
to enter the juice.
Syrup should be kept above 65 solids to minimize use of processing
steam.
Finally, pan operators almost invariably use excessive amounts of
water in the vacuum pans. It is difficult to control, but it is possible to
152
boil a strike of sugar without the use of any water. Steamout of pans
after a strike of sugar is often extravagent. In all pans, except those used
for making seed, steaming is only necessary to wash the massecuite from
the dropping valve.
REFERENCES
1
Eisner, J., Basic Calculations for the Cane Sugar Factory. Booker
Brothers, McConnell London, 1958, 24 pp.
153
Chapter 20
INTRUMENTATION
Instruments can be at once the most useful and the most troublesome
components of the many unit operations in a factory. Unreliable
instruments are worse than none; nonworking instruments are useless. The
basic rules for application of an instrument are:
1.
Put in only devices that the staff can maintain on a 24-hour basis
and repair or replace immediately.
2.
Keep control instrumentation simple.
The main purpose of instrumentation is to improve efficiency.
Reducing manpower may result, but if the factory is already on a
minimum manning basis, added manpower will be necessary to maintain
the instruments.
INDICATING INSTRUMENTS
Common instruments used to show conditions in the system are those
measuring temperature, pressure, flow and level.
More specialized
instruments are used to indicate density, consistency, pH, refractive index
and electrical conductivity. These may be either visual or recording.
Recording instruments are essential for all critical measurements so that
a complete record is obtained. In this way, malfunctions can be traced
and appropriate corrective action taken.
The most important consideration in the use of these instruments is
the location of the sensing element. It must be at a point that is
representative of the condition being measured. It often occurs that the
stem of a thermometer, for example, is too short to reach into the body of
the vessel or pipe. False readings result. Temperatures in a large vessel
like a vacuum pan or evaporator cell vary substantially from spot to spot.
Locating the thermometer, therefore, is all important.
Temperature
Most commonly used thermometers for the lower ranges are glass stem
and bimetallic types. Thermocouples are used for higher temperatures.
Glass stem thermometers depend upon the expansion of a liquid. They
are simple, reliable and low cost. They have the disadvantage of breaking
easily. Also, they are often difficult to read, but one can tell at a glance
whether they are broken.
Bimetallic thermometers depend upon the differential expansion of
two metals bonded together. When formed into a helix, which can be
fitted into a stem, a winding or unwinding motion takes place with a
change in temperature. The movement can be used to move a pointer on a
154
dial. These instruments have the advantage over glass stem thermometers
in their ease of readability, and are less subject to breakage. The
accuracy is not as good and they are subject to change in calibration with
usage. Since a small change in calibration may not be noticed, the
reliability is not as good as with glass stem thermometers.
Thermocouples function on the basis of a change in electromotive
force caused by heat at the junction of two dissimilar metals.
Thermocouples can be used up to very high temperatures and, so, are the
chosen instrument in this area. They are reasonably accurate, easily read
and rugged. They require frequent calibration.
Pressure
Pressure gauges are usually bourdon, diaphragm or bellows mechanical
types, although manometers are used to some extent in vacuum systems.
Since the mechanical types are subject to change with use, they must be
checked routinely.
The bourdon gauge is the most commonly used in the range of 2 to
2
7000 kg/cm^ (30 to 100000 lb/in. ). Its action depends upon the difference
in pressure between the inside and outside radii of a circular, spiral or
helical tube, which causes the tube to tend to straighten when pressure is
applied. Precision measurements are not possible with this action at low
pressure, so they are not useful in systems like exhaust steam lines,
evaporators and vacuum pans.
Diaphragm gauges depend upon the deflection of a flexible disc under
pressure for their action. The most useful range is from a vacuum to 14
2
2
kg/cm (200 lb/in. ), covering that for which the bourdon is unsuited.
Bellows gauges depend upon the expansion of a bellows under pressure
2
for their action. Their range is from vacuum up to 140 k g / c m (2000
2
l b / i n . ) . In general, they are not as reliable as the bourdon and diaphragm
types are, so they are used less. The main disadvantages of the
mechanism are changes in the bellows with use, because of work
hardening, and the fact that the bellows are temperature sensitive.
Flow
Measurement of flow is the least satisfactory of all routine
instrumental measurements. In fact, measurement of the flow of some
process streams
has thus far defied
even reasonably
accurate
measurement.
There is, nevertheless, a wide variety of measuring devices available.
For general factory use, however, the choice is usually between pressure
differential instruments and rotameters. Others that are sometimes used
are weirs, magnetic flowmeters and metering pumps.
Orifice meters are the most common type of flowmeters. The
principle involved is measurement of the differential pressure across an
orifice plate placed in a pipe of flowing material. They are suitable for
measuring both liquids and gases over a wide range of flow rates,
depending on pipe size. Accuracy is highly variable, but rarely is better
155
than 2%. Limitations are many. Of course, they cannot be used for
liquids containing insoluble solids as such will collect at the orifice plate.
Entrained gases can cause difficulty. Variations in density cause errors.
For these reasons, orifice meters are useful in the factory only for
measuring water.
Rotameters consist of a tapered tube with a float whose position is
governed by the flow rate of the liquid or gas. They are useful in a wide
range of flow rates from 0.01 ml to 15000 liters per minute. The
accuracy again varies widely but cannot be expected to be better than 2%
and usually is not that good. The rotameter tends to be self-cleaning so
will handle particulate matter of small size.
Although small size instruments are sensitive to density and viscosity
changes, the larger instruments are less sensitive.
In general, the use of rotameters in the factory is limited to
measurements of relatively small water flow rates.
Weirs are useful for measure of large flows in open systems, such as
inflow of supply water and disposal of waste water. They are really headtype flowmeters based on the principle that the flow rate is proportional
to the head. They consist of a dam across the flow with an opening
through which the liquid flows. The level of liquid in the opening gives a
measure of the flow rate. Accuracy can be as good as 2%, but 5% is more
common.
Magnetic flowmeters are based on the principle that motion at right
angles between a conductor and a magnetic field will develop a voltage in
the conductor. Thus, the liquid under measurement must have electrical
conductivity to act as the conductor. The devices have the advantage
that they introduce no obstruction to flow and require no special piping
arangement. They also are insensitive to viscosity and consistency
changes.
They have the disadvantage that the pipe carrying the liquid must
always be full and any entrained gas is measured as liquid. It is mainly for
this reason that the magnetic flowmeter has not proven useful in the
factory.
Metering pumps are positive displacement pumps that are customarily
used to feed a liquid at a determined rate into a process. They can be
used, however, to measure a flow rate with reasonable accuracy. There
are several types of pumps available, the most important consideration
being that no internal leakage occurs in the pump. Good maintenance,
therefore, is important.
Level
There are a number of types of instruments available for level
detection. Aside from float indicators and sight glasses, the most useful
instruments are differential pressure-type detectors. In these, the
hydraulic head is measured by sensing the pressure difference at the
bottom of the vessel with that in the space above the liquid. Commonly
used sensing elements are force balance and dry motion bellows types.
156
Proximity gauges are used to some extent.
Level detection does not usually present a problem. For that reason,
simple devices suffice.
Specific Purpose Instruments
These include pH, density, consistency, refractive index and electrical
conductivity as the most common instruments.
pH measurements are essential for juices and syrup. The glass
electrode is universally used in conjunction with a calomel reference
electrode to measure electromotive force. The glass electrode must be
handled with care as it is fragile. Being of high resistance, it is also
subject to insulation failure. Another factor in operation is that electrical
fields will change the potential between the electrodes, so screening of
the electrical field is necessary.
Since the electromotive force measured is affected by temperature,
the pH meter must be temperature-compensated.
Generally, acid
solutions have a positive temperature coefficient and alkaline solutions a
negative coefficient.
Most sugar solutions cause scaling of the
electrodes. The only satisfctory solution to this problem is periodic
cleaning. Careful washing with hydrochloric acid is effective.
Density of syrup and molasses is usually observed by means of
hydrometers or hydraulic head devices (similar to those for determining
level). The main difficulty with molasses measurements is the effect of
gas bubbles which can be a large factor. For this reason, refractive index
measurement is a more reliable method.
Consistency indicators are useful in sugar boiling operations. The most
useful of these are rotating probes driven by constant speed motors from
which the torque output is a function of cosistency. Special care is
necessary to maintain a constant shaft friction. This is obtained by means
of a water seal.
Also, flow patterns in the massecuite affect the readings. Location of
the probe in the pan should be such that the flow patterns are relatively
constant.
Electrical conductivity indicators are also used in sugar boiling
control. These measure the specific conductance of the material between
two electrodes. The measuring circuit is an alternating current
Wheatstone bridge. The electrodes are most often made of stainless
steel.
Since conductivity varies with temperature, a temperature
compensator may be used although as temperature rises, viscosity
decreases. This compensates to some extent for the temperature increase
as far as sugar boiling control is concerned.
A major problem is scaling of the electrodes. They must be cleaned
periodically to maintain reliable readings.
157
CONTROLLERS
A system of instrumental control of an operation consists of a sensor
to detect a condition and a transducer to convert the signal to a useful
one and transmit it to a controller. The controller compares the signal
with the desired (set point) and then transmits a controlling signal to a
condition-varying mechanism such as a valve. The most important
component, the sensing element, has already been discussed under
Indicating Instruments.
Only the basic considerations in the other
elements of a control system will be discussed here.
There are two instrumentation control systems - pneumatic and electronic. Pneumatic instruments are favored for a sugar factory because of
ease of maintenance, relative ruggedness and simplicity. Although many
electronic instruments are set up in modular form, a repair facility
requires an expensive stock of spare modules. Even with modular parts, it
is often difficult to locate malfunctions in electronic systems.
Signals in a pneumatic system are transmitted by varying the air
2
2
pressure. The standard range is 0.2 to 1.0 k g / c m (3-15 lb/in. ) which
would correspond to a 0 to 100 control range.
In a typical system, a process transmitter, receiving a signal from a
sensor, transmits a proportional air pressure to the receiving controller.
The controller compares the signal with the set point and sends back an
adjusting air pressure signal to the control element. The mechanism
consists of two pairs of opposed bellows at the respective ends of a force
beam which rests on a movable pivot. At one end of the force beam is a
flapper which changes the back pressure on the detector air nozzle of a
pressure regulator (booster). In the four bellows, the air pressures are
those of the set point, sensor signal, reset and feedback functions. The
force beam will be in equilibrium when the forces from all four bellows
come to balance. A change in force on one of the bellows causes beam
movement and a change in the position of the flapper. This causes a
change in the feedback pressure, restoring the balance.
Unit operations where controllers are essential are:
1.
2.
3.
4.
5.
6.
7.
8.
Mill and diffuser cane feed.
Boiler pressure, excess air draft, water feed and safety interlocks.
Clarification liming pH.
Juice heater temperature.
Evaporator vacuum, feed, level, syrup density and vapor loop
pressure.
Vacuum pan, vacuum and feed.
Crystallizer temperatures.
Centrifugal sequence cycle.
In choosing control instrumentation for these operations, it is wise to
standardize as much as practicable in order to reduce the complexity of
spare parts inventory. Recorders are advisable on all these basic
controllers.
158
OPERATION
The most important single factor in the use of instruments is that the
operator has confidence in their value. This means that the instruments
must be reliable. Not only must they indicate actual conditions but also,
it is necessary that there be immediate indication of a malfunction.
The job of the operator should be limited to using the instruments - not
servicing them. Servicing should be in the hands of a qualified technician
who is available immediately when instrument failure occurs. The
technician requires a stock of parts so that repairs can be made at once.
An inventory of spare instruments is necessary for particularly vital
control devices.
For pneumatic systems, since clean air is of prime importance, there
should be an air compressor used only for this purpose. Daily monitoring
of the air quality should be made.
Electronic controllers must be housed in dust-proof
Likewise, they should be isolated from heavy vibrations.
compartments.
159
Chapter 21
EQUIPMENT MAINTENANCE*
EFFECTIVE MAINTENANCE
High time efficiency, good sugar recovery and low production costs
necessitate an effective maintenance program. Good maintenance is also
reflected in good factory housekeeping. Cost effective maintenance is
the result of planning and effort by all personnel.
Maintenance is on-going and not a project with a start and completion
date. Over a period of time individual factories have developed their own
systems and procedures for maintenance. The elements common to
effective maintenance programs are the following:
Records: Complete data on equipment including purchase orders,
specifications, drawings, operating manuals, maintenance instructions,
spare parts lists and operational history.
Inspection program:
equipment.
Work order system:
maintenance.
Schedules
and
procedures
for
inspecting
Procedures for organizing and accomplishing
Spare parts: Controls to maintain
critical parts.
inventory of fast
moving and
Investigation and analysis: Investigations of equipment stoppages and
breakdown and analysis to prevent reoccurrence.
Equipment and technology: Evaluation of new equipment
technology, as possible replacement of high maintenance items.
and
Supervision and training: Supervision to insure proper maintenance and
training to improve skills.
A program of scheduled inspections and repairs is an important part of
maintenance control. Judgment by experienced personnel is the key to
the program. The balance between too much repair and not enough is
elusive and practical judgment is necessary to execute the program to
minimize stoppages while controlling production costs. A daily, period
and annual off-season schedule should be set up and followed. All
inspections and repairs should be recorded.
A daily inspection by trained personnel walking through their area
visually checking the equipment, touching motors and bearing housings,
listening to pumps and gear boxes, and doing something about this
information is vital to reducing lost time. Most breakdowns do not happen
John W. Herkes assisted in the preparation of this chapter.
160
suddenly, but are caused by wear over a period of time and often this wear
can be detected visually or by a change in temperature, sound, or
vibrations well ahead of the actual breakdown. This prior warning, if
acted upon, usually results in less damage and gives the staff time to
prepare for the work beforehand.
Many minor repairs can be
accomplished on the run instead of waiting for scheduled shutdowns.
Period repairs made during scheduled shutdowns include routine
maintenance and jobs which have been shown necessary during the daily
inspections. A work sheet lists these and shows priorities.
The annual off-season maintenance schedule should be planned
carefully, charted, and status followed. The chart should include job
identification, estimated man hours, and personnel assignments.
GUIDELINES FOR MAINTENANCE SCHEDULING
Conveyors
After the first month of operation, new conveyors should be inspected
for abnormal wear. This inspection is important as it forms the basis for
any corrective action which may be necessary.
While operating, check heavy duty conveyors hourly for broken links
and missing pins. Medium and light duty conveyors may be checked once a
shift.
During the scheduled shutdown
replace worn or bent slats and pins.
make
a complete inspection
and
During the annual shutdown overhaul completely, replacing worn chain
links and pins, slats, wear strips, idlers, bearings and sprockets. Keep
records of these repairs as this information is helpful for future purchase
as well as scheduling overhauls.
Knives
While operating check the external area hourly, particularly for
vibration and hot bearings. Take advantage of the longer mill delays by
stopping the knives and replacing broken blades giving first consideration
to balancing the set.
During the scheduled shutdown inspect knives for wear and replace
worn ones.
During the annual shutdown replace
bearings, couplings and lubrication system.
knives,
worn hubs,
inspect
Shredder
While operating check hourly for vibration and hot bearings.
During the scheduled shutdown inspect hammers and grates for wear
and replace with built-up sets as required before excessive wear takes
place. Check lubrication system and grease hammer bushings.
161
During the annual shutdown replace hammers and grates. Inspect the
bearings and couplings and change the oil and grease in the lubrication
system.
Mills
While operating check hourly. Inspect intermediate carrier chains,
hydraulic system and lubrication system. Check bearing temperatures on
mills and gearing. Lubricate immediately squealing bearings. Once a
shift check for loose hook bolts, eccentric pins, bearing wedges and
shims. Build up grooves of rolls as necessary. Mild steel or hard facing
rod may be used.
During the scheduled shutdown clean the mill thoroughly, check and
adjust the mill openings, check the turner plates and inspect the underside
of the mills. Inspect condition of the mill grooving, repairing any areas
damaged by tramp iron and build up grooves. Check lubrication and
hydraulic systems. Inspect intermediate carriers and pumping systems.
Test overspeed safety tripout on turbines while shutting down for
weekend. Schedule replacement of worn rolls and turner plates.
During the annual shutdown, dismantle the mills for a complete
overhaul. Inspect the cheeks, foundation bolts and juice pans. Install new
rolls or regroove existing rolls, replace turner plates and scrapers and fit
all bearings. Overhaul all carriers and chutes. Overhaul cush cush
screens, pumps and valves.
Check nitrogen pressure on Edwards
hydraulics. Clean lubrication system. Check gearing for wear and
alignment. Check foundations for settling.
Diffuser System (Silver)
Buster and Fiberizer
While operating check the oil temperature
bearings hourly and the heat exchanger daily.
of the
main
rotor
During the scheduled shutdown inspect hammers and grates for wear
and replace with built-up sets as required before excessive wear takes
place. Grease bushings on each hammer. Check the lubrication systems.
During the annual shutdown replace the hammers and grates.
Inspect the bearings and change the oil and grease in the lubrication
systems.
Diffuser
While operating check the entire lubrication system daily. Inspect
all pumps daily.
During the scheduled shutdown inspect the entire lubrication system
and supply required oil and grease. Inspect all conveyor chains, sprockets,
idlers and slats. Check the rubber belt and idlers. Oil the equalizer
chains. Inspect V-belt and couplings on hydraulic drive. Clean and check
juice diverter and clean activating cylinder. Check distribution drain
plugs. Inspect each of the lower discharge screw bushings at least once a
month. Inspect the discharge screws and build-up with hardfacing as
required.
162
During the annual shutdown inspect all bearings and couplings and
change the oil and grease in the lube system. Calibrate Weightometer and
check counter weight assembly. Inspect all rubber seals at sprockets,
levellers and idlers.
Dismantle and inspect the discharge screw drive couplings and the
two flexible couplings above the speed reducers. Inspect the bearings,
seals and gearing on the speed reducers.
Inspect the power unit, cylinder, valves, filter and piping on the
diffuser feed plate. Replace rubber side seals.
Change hydraulic oil on the hydraulic drive.
Boilers
Refer to:
(a)
(b)
American Society Mechanical Engineers (A.S.M.E.) Boiler and
Pressure Vessel Code VII, Suggested Rules for Care of Power
Boilers.
Instruction Manual for boiler provided by the manufacturer.
Shut down a new boiler for a few days during the second month for a
complete inspection.
Check the furnace refractories, grate, tubes
externally for movement, erosions and soot deposits, drums internally for
scale, baffles for looseness or wear, and the air and hot gas systems for
abnormalities.
During the annual shutdown, inspect boilers with special attention to
the following points:
1. Any metal in the furnace for breakage or burning such as grate
bars, links and supports.
2. Refractories opposite oil burners, adjacent to doors and small
openings, and near the bottom of the furnace when subjected to damage
from cleaning tools.
3.
Inside walls and outside walls for excessive bulging.
4. Tubes (outside) for sagging, erosion, corrosion near the lower drum
and soot deposits. Tubes (inside) for scale and plugging.
5. Drum (outside) for corrosion and drum (inside) for scale, corrosion,
and loose tubes.
6.
Support steel for sagging and burning.
7. Check level indicators, baffles, tuyeres, oil burners, soot blowers
and flues, relief valves and fusible plugs.
Make the necessary repairs and also anticipate the major repairs for
the following year. This allows time to order material and plan the
projected work load.
Turbines
Inspect the turbines daily.
163
During the annual shutdown:
First Year: Remove top casing and inspect internals.
glands, seals, bearings, valves and governor.
Check blading,
Second Year: Complete inspection and overhaul.
Following Years:
every 2 to 4 years.
Minor inspection
each year.
Complete overhaul
This applies to all turbines in the factory, but the annual minor
inspection varies depending on the type of turbine and duty conditions.
The three major classifications are:
1. Powerhouse Turbines: minor inspection would cover bearings,
glands, governor and blading if machine is equipped with an inspection
port.
2. Mill Turbines: minor inspection would cover bearings, governor
and the valves which receive heavy service.
3. Pump Turbines:
minor inspection consists of checking the
bearings and governor. These simple rugged machines require a minimum
of maintenance.
Consult manufacturers manuals for specific recommendations.
Generators
Twice a shift log information from a visual inspection and temperature
readings.
During the annual shutdown:
First Year: Complete inspection and maintenance.
Following Years: Minor inspection each year. Complete overhaul
whenever the drive unit is overhauled; at least every 3 years.
When not in service, keep covered and use space heaters.
Electric Motors
All electric motors require a daily visual inspection and a temperature
check.
During the annual shutdown:
High contamination area: should be opened and cleaned every year.
Moderate contamination area: should be checked
opened and cleaned approximately every two years.
each
year
and
Clean area: should be checked every year and opened and cleaned
every three years.
During the off-season, large motors should be covered and protected
from moisture with space heaters.
164
Gear Reducers
Mill and Powerhouse
Twice a shift, visual inspection and temperature check should be
made. On a new installation, check teeth alignment and general
conditions after one month of use. A t the end of the first year, there
should be an overhaul with special emphasis on teeth wear and foundation
settling.
After the second year, make a complete inspection.
For the following years, have a minor inspection with a complete
overhaul every third year.
Conveyors
Daily visual inspection and temperature check.
Minor inspection every year.
Complete overhaul every third year.
Scales
Before operations begin, all scales should be dismantled, repaired,
cleaned and adjusted.
Beam scales must be calibrated with known
quantities.
Make a visual inspection and check the tare on beam scales each shift.
As very little can be done on a daily basis for the fixed quantity type
scales, schedule another filling test for the second half of the year.
Whenever welding in the area of scales, be sure the ground lead is on
the section being welded to prevent any current flowing through the knife
edges.
Heaters
Check condensates for sugar contamination.
During shutdowns, clean the tubes by circulating hot caustic solution
(over 40%).
After the cleaning a simple static head water check is advisable. This
can be done by pumping water through the heater and opening the
condensate line to check for water.
During the annual shutdown, inspect the tubes making the necessary
replacements, change gaskets and seals and before closing check with a
pressure test.
Clarifiers
At the end of the season, after liquidating the clarifier, immediately
fill with water until time is available for cleaning. Early cleaning is
advisable but if delayed, add lime or caustic soda to the water.
Check scrapers, arms, shaft guides, trays, door seals, gas vents and
make the necessary repairs. Paint an epoxy band approximately three
165
feet wide at the juice level to prevent corrosion caused by a fluctuating
level.
Filters
Daily: Check screens, scrapers, juice passages, vacuum gauges and
general conditions. Broken screens should be repaired or replaced as soon
as possible to prevent plugging. Clean water spray nozzles.
During the annual shutdown, make a complete inspection and repair
any damaged screens. Replace the scraper and check the trunnions,
tubing, deck plates and agitator. Periodically, a major overhaul is
required changing tubing, deck plates, valves and seals.
Evaporators
Check condensates for sugar contamination. Clean the tubes during
the weekly shutdown circulating hot caustic soda (over 40%) followed by
inhibited (ferric sulfate) sulfamic acid. It is important to wash the vessels
free of sugar before adding the caustic. It is also important to stop all
water leaks into the system during cleaning as this will dilute the caustic
solution.
After cleaning make a simple static head water check. This can be
done by installing a wing-bolt type manhole on the bottom of the cell.
Open this manhole and after the steam chest is completely full of water,
check for leaks.
During the annual shutdown, replace the worn and leaking tubes, check
the save-alls and feed inlet pipes, inspect domes and connecting piping,
especially if constructed of mild steel, replace door gaskets and conduct a
pressure test. Clean the tubes on the vapor side with alkaline
permanganate followed by acid ferrous sulfate, or proprietary compounds.
Pans
Check condensates for sugar carryover.
When the tubes become dirty, boil for one-half to one hour, with water
or sulfamic acid between strikes. Additional cleanings with acid or
caustic soda during the periodic repair stops should be standard practice.
After cleaning, make a static head water check for leaks as with
evaporator bodies.
During the annual shutdown, check and replace all bad tubes, check the
save-alls and feed inlets, inspect the domes and vapor pipes - especially if
constructed of mild steel - check manhole gaskets and pressure test the
calandria. Clean the tubes on the vapor side as with evaporators.
Centrifugals
Semi-Automatic Batch
Daily inspection of screens, drives, brakes and general operating
conditions.
Gear driven batteries
require additional checks and
adjustments of the friction bands every week.
166
During
overhaul the
cylinders. A
machines are
the annual shutdown, inspect the machines thoroughly and
braking system, rubber buffers and governors and actuating
complete overhaul should be made every two years or, if the
old, every year.
Automatic Batch
Daily visual inspection of screens, drives, braking, hydraulic and
pneumatic actuators and general operating conditions. It is important
that the electrical components (relays, contacts, timers, etc.) be kept in a
clean atmosphere.
During the annual shutdown, inspect the machines thoroughly and
overhaul the braking system, rubber buffers, hydraulic and pneumatic
actuating cylinders, solenoid valves, relays, timers and contacts. A
complete overhaul should be made every two years.
Continuous
Daily visual inspection of the general operating conditions. A good
time for this is in conjunction with the screen washing. Then, with a
strobe light, the rotating basket can also be checked before opening the
feed valve. Check molasses from each machine every shift for sugar
crystals which would indicate a broken screen.
The main basket bearings must be oil-protected during shutdowns.
Therefore, on some designs, the oil pump must be run daily for a few
minutes.
Check screen clamping ring to insure that the screen fits tightly
against basket to prevent screen flexing and breakage. If basket is worn
turn down the ring to the same size.
Every year the machines should be thoroughly inspected, in
particular the baskets for stress corrosion. Perform a complete overhaul
every two years.
Crystallizers
Daily visual check of the crystallizers plus a water test of the empty
units looking for leaks. Repair any leaks at once.
During the annual shutdown, half of the crystallizers should
emptied, inspected and the necessary repairs completed.
be
Pumps
On a new installation, open up the pump for an inspection after one
week to one month of operation depending on the amount of wear
expected. This inspection will be the basis of the repair program.
All pumps should be visually inspected daily for abnormal sound and
temperature rise.
Many pumps utilizing alloy steel shafts and impellers on nonabrasive
service can operate two or three years without an overhaul. However,
during the annual shutdown, take the cover plate off and check the
impeller, casing and shaft play. Where the duty is severe, an annual
167
overhaul is in order. In very severe cases, complete inspections and
impeller changes are required during the season.
Tanks
Tanks should be washed out during the annual shutdown, then
inspected. Tanks holding alkaline or near neutral liquids normally need
not be painted. With good paints many tanks will go two or three years
without repainting.
Piping
Most piping in the factory can be
shutdown, after flushing out with water
exception is the syrup piping. In most
scraped clean. Periodic washing with
cleaning will make this unnecessary.
left in
at the
cases,
caustic
place during the annual
end of the season. The
this must be opened and
soda during evaporator
Valves
Most valves should be taken out, repaired and tested by a simple static
water test during shutdown. Valves in severe service should be checked
more often.
Fans
T
On a new installation a fan should be checked after a month s
operation to determine wear. In severe wear cases, the blades or the
complete rotor may have to be replaced periodically during the season.
During the annual shutdown, inspect the blading, housing and bearings,
making all necessary repairs.
Instruments
Automatic controls, properly installed, will function only with proper
care. Basic rules must be adhered to or control will soon become
unsatisfactory. Refer to the instrument manuals for specific instructions.
An adequate supply of clean air is essential for pneumatic
instruments. Clean air means free of foreign particles as well as oil and
water. A separate oil-free air supply system for instruments is
recommended.
Pneumatic
No repair work of any kind should be done by unauthorized personnel.
Perform the routine maintenance that some installations
such as cleaning pH electrodes, screens and diaphragms.
require
At least once a week, the instrument should be inspected, cleaning
the air filters, moisture traps and pens. A t the same time check the
system while raising and lowering the set point and air regulator.
During the annual shutdown, clean and inspect the instruments
paying special attention to the ink system, control relay intrument
balance - zero and span - and tubes. Check all tubing for air leaks and
plugs, and drain water from traps.
168
Electronic
Electronic instruments should only be serviced by trained electronic
technicians. For this reason the technician must be available at all
times. An inventory of essential components, particularly of modules,
must be maintained. Electronic instruments must be kept in a dust-free
and vibrationless cabinet.
169
Chapter 22
GENERAL RULES OF THUMB FOR APPRAISING F A C T O R Y
OPERATIONS AND OPERATIONAL TARGETS
GENERAL RULES OF THUMB FOR APPRAISING F A C T O R Y
OPERATIONS
General approximations which are useful in rapid appraisal of factory
operations are listed here. They show the order of magnitude of average
figures and give a clue as to reported figures which are out of line.
Cane (Mature, sound stalk, 24 months)
Fiber %
Pol%
Refractometer solids %
Purity of juice
13
14.5
16
90
Pol = Pol in first expressed juice times 0.8 (Java Ratio).
3
3
No void density (for milling) 1121 k g / m (70 l b / f t ) .
Tops (At break point, mature unburned cane, 24 months, dry conditions)
% of cane
Fiber %
Pol%
Refractometer solids %
Purity of juice
5
22
4.5
9
50
Fibrous Trash (full burn, dry weather, including dead cane)
% of rake harvested cane
7
Fiber as harvested %
Dry conditions
70
Wet conditions
30
Bagasse
Pol = Pol in last expressed juice times 0.75 (mill).
Density
Fresh
Stored
3
3
112 k g / m ( 7 l b / f t )
3
3
160 k g / m (10 l b / f t )
Fuel value changes 1% for each 1% change in bagasse moisture.
Extraction changes 0.1% for each 1% change in bagasse moisture.
Mixed Juice
Purity = Purity first expressed juice minus 2.5.
Purity = Purity disintegrator extract of cane minus 1.5.
170
Filter Cake
Tons
Tons mixed juice
% insoluble solids χ 4
plus tons mixed juice times
100
100
Tons bagasse fines required equals tons insoluble solids in mixed juice.
Syrup
Purity = Purity mixed juice plus 0.5.
Massecuites
3
3
Weight of A and Β = 1.56 m /metric ton (50 ft /short ton) 96 DA sugar.
3
3
Weight of low grade = 0.34 m /metric ton (11 ft /short ton) 96 DA
sugar.
Low Grade Sugar (Remelt)
Final molasses recirculated at 80 purity is twice that at 90 purity.
Sugar
3
3
Density commercial raw = 881 k g / m (55 l b / f t ) .
Attenuation index at 99 pol = Attenuation index crystal times 3.
99 pol contains 1.3% molasses.
Final Molasses
Refractometer
plus 5.
sucrose purity = Refractometer deleaded pol purity
Refractometer solids = 1 to 1 spindle brix minus 2.5.
Quantity changes 2.5% for each change of 1 point in syrup purity.
Viscosity doubles for each 10°C drop in temperature.
Viscosity changes 50% for each unit change in refractometer solids.
OPERATIONAL TARGETS
A summary of general operational targets for factory operations is
listed here. The targets are intended to be used only as guides and
operating conditions must be taken into consideration.
Cane Preparation
Displaceability index
Mill
Diffuser
87
94
Imbibition
% of fiber milled
250
171
Bagasse
Pol%
Mill
Diffuser
2
1
Last Expressed Juice
Refractometer solids %
Mill
Diffuser
3
1.5
Filter Cake
Pol%
1
Syrup
Refractometer solids %
68
Massecuites
Refractometer solids %
Purity
Crystal content
A
Β
C
92
85
50
92.5
75
40
96
58
35
Sugar
Pol%
Moisture %
Crystal size mm
Temperature ° C (maximum)
99
0.2
0.8
40
Final Molasses
Refractometer solids %
Purity above expected
Temperature ° C (maximum)
83
6
40
Undetermined Loss
Pol%
1.7
173
Chapter 23
CANE
STRUCTURE
The stalk of the sugar cane plant is a two-phase system, solid and
liquid. The solid phase is a cellulose-lignin-pentosan complex known
generally as fiber. The liquid is a water solution, the juice, containing a
large variety of organic and inorganic substances, close to 90% of which is
sucrose.
The structure of the stalk is shown in the cross sectional drawing of an
internode (Fig. 23-1). The hard rind encloses a matrix of thin-walled
parenchyma cells in which are imbedded the vascular bundles. The rind
and the vascular bundles constitute what is commonly called the "fiber" of
the fibrous portion while the sheet-like parenchymatous tissue is referred
to as "pith." There are thus two types of materials which make up the
solid phase of the stalk reported as fiber in the customary methods of
analysis.
The parenchyma tissue forms the walls of storage cells which hold high
density, high purity juice. Inside the relatively sturdy vascular bundles are
the conducting vessels which extend throughout the plant from the roots
to the leaves. Through this piping system, water and nutrients move from
the roots to all parts of the plant and products of photosynthesis
translocate from the leaves. The fluid in this conducting system is,
therefore, highly variable in composition and is lower in purity and more
dilute than the juice in the storage cells. Being protected by the thickwalled fiber bundles, it is not so readily expressed as the juice in the
storage cells. There are, therefore, two types of liquid - juice in the
storage cells, which might be called static juice, and juice in the
circulatory system, which could be called transit or dynamic juice.
The vascular bundles are closer together at the rind and in the nodes so
the fiber content is higher. Also the juice in these regions is lower in
purity and sucrose content because of larger amounts of vascular juice.
The range in analytical values for a cane averaging 12.5% fiber, 15.5% pol
and 90 purity is as follows:
Internode
Rind section
Center section
Node
Rind section
Center section
Fiber
Pol
%
%
Purity
20-25
5.5-6.5
12-14
17-19
87-89
91-94
13-14
11-12
10-10.5
13.5-14.5
75-76
86-88
174
Fig. 23-1. Cross section of cane stalk.
175
It must be kept in mind that these are analyses based on separation of
components by rough methods. Actually, the composition within the
growing plant is unknown because the components are altered in the
process of separation. For example, the juice expelled from a stalk of
cane by the application of pressure usually has a pH in the range of 5.2 to
5.5. Some mechanism is active whereby sucrose can be stored at this pH
for long periods of time without inversion.
There is evidence, again from separated materials, that some of the
water is not affiliated with the juice but with the fiber - the so called
bound water which may be 20 to 25% of the weight of the fiber.
JUICE
The total juice in the cane is referred to as Absolute Juice or Normal
Juice. In most factory control systems, the quality of this juice is
calculated, based upon the analysis of mill juices. The assumption is made
that the total juice in the cane consisted of the sum of the juice
extracted, as appearing in Mixed Juice, and that remaining with the fiber
in the bagasse. Furthermore, the juice remaining in the bagasse is usually
assumed to have the same composition as that of Last Expressed Juice
(juice obtained in dewatering bagasse). Since Last Expressed Juice is
always of lower purity than Mixed Juice, the calculated purity of Absolute
Juice is always lower than that of Mixed Juice.
With the advent of direct analyses of cane and subsequent return in
many factories of processing of cane by diffusion rather than milling,
calculated figures for Absolute Juice were no longer necessary.
Substantial differences were then noted to exist between calculated and
measured values. Reasonable certainty exists that the composition
calculated from Mixed Juice and Last Expressed Juice does not represent
the original juice in cane, nor is that found in the direct analysis of cane
the same. In the intact cane plant, juice and fiber have an equilibrium
homogeneity. When separated, both change.
Mixed Juice is all the juice recovered in the extraction plant. Its
composition depends upon the quality of the cane, the efficiency of the
extraction process and the changes occurring under the conditions of
extraction. The changes are largely unknown, at least not established, but
they indicate those caused by microbiological action, chemical changes
such as inversion, and solubilization of solid material. The extent of these
changes is variable but investigation has shown that Mixed Juice from a
milling plant is about 1.5 points lower in purity than juice in cane as
determined by disintegrator analysis.
Tests on diffusion plants have shown similar differences. In this case,
it has also been established that there is an increase in soluble solids in
the diffuser, indication that solubilization is taking place. In the diffuser,
there is no biological activity, but some chemical inversion can occur
having an effect on purity.
It is also a fact that in the process of direct analysis of cane by
disintegration, some changes occur. So, it must be concluded that the
176
original juice in the cane is higher purity and lower soluble solids content
than routinely reported.
The juice is also affected by the quality and quantity of the trash in
the cane entering the extraction plant. Cane tops in particular have a
significant influence because they contain expressible juice of low purity
(around 50). Even dry leaves, although containing no expressible juice, are
subject to leaching, and nonsugars, especially colored substances, enter
the juice.
The result of all these effects is that the quality of juice from which
the factory must make sugar is lower than that of the growing cane stalk.
FIBER
The two types of fiber - fiber bundles and parenchyma - differ in
physical properties but chemically they are similar. The fiber bundles are
long, tough and stringy. The parenchyma (pith) is thin, weak and
paper-like. The fiber bundles, providing the structural integrity of the
cane stalk, are concentrated mainly in the rind, while the parenchyma is
the internal cell wall material. The relative quantities of the two vary
therefore with the age of the cane, stalk diameter, growth pattern and
variety. It is true also that the strength and toughness of the fiber
bundles varies with these and in particular with variety. Some canes have
strong fibers, others brittle and readily fractured ones.
The disintegration and milling properties of a cane change with the
quality of the fiber. A strong-fibered cane may be difficult to
disintegrate but feeds well in a mill because of the long fibers. A weakfibered cane will disintegrate readily but has poor feeding characteristics
because of the short and sometimes powdery fiber.
VARIETIES
Commercial cane varieties are bred primarily for optimum yields of
sugar. The breeding program has led to canes which are at a relatively
constant yield level and which do not vary much in composition. The fiber
content, juice solids, purities and general characteristics are close to the
same. The geneticists effort therefore is mainly in the direction of
producing disease resistant varieties and new clones to replace those
suffering from yield decline. So in general varieties do not play a major
role in factory processing. The varietal characteristics which do show up
are usually in milling where it is possible to make adjustments to
accommodate fiber quality differences.
REFERENCES
1
Martin, J. P., Sugar Cane Diseases in Hawaii, Advertiser, Honolulu,
1938, 295 pp
177
Chapter 24
REGIONAL VARIATIONS IN SUGAR CANE PROCESSING
The Hawaiian sugar industry is characterized by:
Administrative cane with few independent growers
Two-year cropping
Year-round factory operations with a shutdown for yearly maintenance
of one to three months
Minimum manpower with 40-hour labor week
Mechanical harvesting, principally by pushrake
Extremely varied rainfall and terrain
Production of raw sugar only
Other sugar regions have some of these conditions, none have all.
Administrative cane, for example, is the rule in Brazil and Peru.
Two-year cropping is the practice in Peru and to a limited extent in South
Africa. Colombia and Peru harvest cane the year around. Peru harvests
extensively by pushrake, a practice which is just beginning in Brazil. No
other area attempts to harvest under heavy rainfall conditions. The sum
of Hawaiian conditions imposes upon the factory the necessity of handling
a robust cane which may contain more than 50% extraneous material tops, leaves, weeds, soil, gravel, rocks and tramp iron. The material
entering the factory, therefore, is not cane but a crude ore varying in
cane richness. A massive cleaning plant becomes a necessity for
preparing the cane for milling and diffusion. These cleaners are the
unique characteristic of Hawaiian factories.
Beyond
the same
effective,
extraneous
the cleaning plant the unit factory operations are essentially
as in other sugar regions. Since no cleaner is completely
however, the succeeding steps must handle a greater load of
matter and equipment is subject to heavy wear.
In the following sections is a comparative discussion of practices with
those in other sugar regions.
CONTROL
By far the greatest amount of sugar cane in the world is grown on a
one-year cycle, harvested by hand and delivered to the factory relatively
free of extraneous matter. Harvesting is done in dry weather and the
factory operates less than six months of the year. Under these conditions
it is possible to start factory control accounting with the weight of cane.
This being impossible in Hawaii because of the large amount of extraneous
material, control starts with the weight and analysis of extracted juice.
The quantity of cane can only be arrived at by calculation and estimation.
This constitutes the principal difference in Hawaiian control methods.
178
Recovery balances are made on the basis of the weights of juice, final
molasses and sugar and the analysis of the streams from samples taken by
continuous or intermittent means. The basic analyses are pol and
refractometer solids. Mixed juice must be analyzed for insoluble solids in
order to correct the gross weight to weight of liquid.
MILLING
Cane milling in Hawaii is characterized by low mill speeds. Extraction
efficiency suffers from high roll wear caused by abrasive soil constituents
brought in by mechanical harvesting. Otherwise, milling is much the same
as in other areas. Major emphasis is on good cane preparation and low
bagasse moisture. Shredders are an essential part of the milling tandem
and are commonly placed after a two-roll crusher. This arrangement
protects the shredder hammers from large rocks and also reduces the
power necessary for the shredders. The most common feeding aid is the
under feed roll, although there are several heavy-duty pressure feeders in
use.
Coarse circumferential grooving is not used because of the adverse
effect on extraction. Although groove wear is severe, periodic arc hardfacing helps in maintenance. Messchaert grooves are standard. A hot
compound maceration system is routine with dilution in the range of 200
to 250 percent on fiber.
DIFFUSION
Present-day basic principles of cane diffusion were etablished in
!
Hawaii in the 1950 s. The importance of proper cane preparation for bed
permeability and high extraction and the advantage of nonmoving cane
bed are features which have been followed. Although the term diffusion is
universally used, the process in Hawaii is recognized by a more proper
description as juice displacement - a process involving the separation of
the juice from fiber by displacement with water.
In several regions a hybrid system of combined milling and diffusion
has been introduced and is referred to as bagasse diffusion. In this, the
cane first passes through one or two mills, then through a diffuser, then
through mills for dewatering. This combination of milling and diffusion
requires extra equipment and runs counter to sound technology. It does
not improve extraction.
The Hawaiian diffusion plant consists of leveling knives, two
heavy-duty shredders in series for cane preparation and the stationary bed
Silver ring diffuser. Moving bed diffusers, which were the principal types
installed in the decade 1970-1980, have some advantage in flexibility over
the fixed bed type. With a moving bed, however, it is not possible to
control the liquid level and avoid channelling, particularly on the sides of
the bed, so efficiency falls below that of a fixed bed diffuser.
Bagasse dewatering remains the major unsolved problem in diffusion.
The original installations in Hawaii avoided the use of mills. The
extrusion screw press and the cone press proved to be effective
179
dewatering devices and were used for many years. Continuing high
maintenance costs led to their replacement by mills. Both of these
machines, nevertheless, are attractive in principle and it is reasonable to
predict that inventive effort will bring the return of something similar
with the obsolescence of massive mills.
CLARIFICATION
Since only raw sugar is produced, simple clarification is employed in
Hawaii. To minimize scaling, it is normal to use a mixture of lime and
magnesium oxide added as a slurry to cold juice. Heating and clarifier
settling follow. Polyelectrolytes are added to the heated juice at a rate
of about.two parts per million.
Similar procedures are followed in other sugar areas where raw sugar
is the product, except that magnesium oxide is rarely used. Liming may
be done on hot juice. Any advantage of this has not been universal and
mechanically it is difficult to perform. Although a wide variety of
clarification aids may be found in use, the economic value of any but
polyelectrolytes is problematical. In phosphate-deficient juice, increasing
the level of phosphate may be helpful, however.
In factories producing a direct consumption sugar (plantation white),
the most common practice is sulfitation. Sulfur dioxide is added to the
juice followed by liming and heating. In comparable procedures, the juice
may be limed partially or completely first, followed by sulfur dioxide
treatment. The remainder of the clarification, settling and filtration,
remains the same as in simple clarification. If a better quality of sugar is
desired, syrup sulfitation is an added step.
SUGAR BOILING
Sugar boiling procedures are universal in principle, but the chosen
system of handling massecuites in many areas appears to follow the fancy
of the technologist. The final result can be the same regardless of the
route involved in going from syrup to final molasses by removing sugar by
crystallization. The efficiency of operation depends upon the extent that
conditions are controlled. Thus instrumentation is a vital necessity and
those factories with advanced control instruments outperform those
depending upon manual control. The problem with instruments in many
areas, however, is the absence of local maintenance facilities, so it
becomes necessary to rely on skilled operators.
Hawaiian cane yields high purity juices making a 2-massecuite boiling
system impracticable, so a standard 3-massecuite system is used.
Material flow is contercurrent with flow of low purity effluent toward
final molasses. Α-strikes are boiled primarily on syrup. B-strikes are
boiled on Α-molasses along with the remelted low grade sugar. A l l strikes
are seeded with a slurry of sugar in isopropyl alcohol ground in a ball mill.
A and Β sugars together constitute commerical sugar, which averages
close to 99 pol. No crystallizers are used on commerical sugar. No sugar
dryers are used and sugar is stored and shipped in bulk.
180
Low grade massecuites are cured in water-cooled crystallizers for
relatively long periods (48 to 72 hours). This period is necessary with
characteristically slow crystallizing massecuites from high salt content
juices. Massecuites are reheated to close to the saturation point in
massecuite heaters (up to 60° C) before centrifuging.
In many sugar locales where cane growing conditions are less
favorable, such as those with winter frost, or at the other extreme
low-lying locations near the equator, juice purities are lower. If they fall
below 80, it is possible to use a two-boiling system making only one
commercial strike and one low grade strike. This is often done with
satisfactory recoveries. It has the advantage of simplicity and of
producing only one quality sugar.
The practice of using crystallizers on A - and B-massecuites as well as
low grade is also common throughout the world. Retention time is short
(2 to 6 hours) and often continuous crystallizers are used.
Sugar dryers are common both for raw and plantation white sugars.
STEAM GENERATION AND UTILIZATION
Because many Hawaiian factories must supply considerable power for
the plantation irrigation system and furnish surplus electricity to the
community utility grid, they have been traditionally energy-efficient.
Boilers are high pressure for the cane industry, ranging from 32 to 85
2
2
k g / c m (450-1200 lb/in. ) (3103-8274 kPa) pressure. Most furnaces are
designed to burn the bagasse in suspension, which is possible with the
finely divided bagasse. Air preheaters and economizers are normal.
Boilers are completely instrumented and control is automatic. In most
instances there is only one boiler per factory.
The cooperative tie-in with the public power utility permits the
factory to sell surplus power to the utility and buy power when the factory
is not running. Boilers are equipped to burn oil when necessary. Recent
design changes have been directed toward burning of waste products in
order to reduce oil consumption.
This is in sharp contrast with most cane sugar areas where little or no
2
electricity is exported. Boilers are usually low-pressure, 10 to 16 k g / c m
2
(150-225 lb/in. ), (1034-1551 kPa), and multiple smaller units are employed. Furnaces tend to be hearth-burning. Instrumentation is minimal
with manual control. Supplemental fuel, commonly wood, is burned when
necessary.
Whereas in Hawaiian unit factory operations, close attention is paid to
steam economy; such has not been the rule in many other areas.
Ordinarily, there should be ample bagasse to supply internal factory needs
without much attention to economy. With efficient boilers and sound
boiling house steam utilization, there should be an excess bagasse of the
order of 25 percent.
181
MANPOWER
Hawaiian factories are noted for the efficient use of manpower.
Workers are employed the year-round whether the factory is operating or
not. Their retirement benefits are close to their pay while working. The
standard work-week is 40 hours, so factories normally operate on a 5-day
week.
Factory operations are automated as much as practicable making
manpower requirements minimal. Total manning, including shops, for a
factory processing 200 tons cane per hour would average 120. Actual
operators per 8-hour shift would be under 20. Such a system requires
close attention to maintenance of equipment. Outside contractors are
utilized extensively for major repairs and new installations.
183
Chapter 25
ENGINEERING
SPECIFICATIONS
Factory installations should be designed to conform to accepted
engineering standards.
These standards are well established and are
updated periodically. In the United States the essential standards are those
of:
American National Standards Institute (ANSI)*
2
American Society of Mechanical Engineers (ASME)
American Society for Testing and Materials (ASTM)
American Petroleum Institute (API)
American Welding Society (AWS)
National Electrical Manufacturers Association ( N E M A )
Other manufacturing countries have analogous and usually equivalent
standards.
Although the design specifications for each piece of equipment must be
detailed to fit the individual requirements, all are subject to general
standards.
The most basic of these standards are summarized in this
chapter.
VESSELS
Vessels include non-pressure tanks and unfired pressure vessels such as
juice heaters, evaporator bodies and vacuum pans.
Non-Pressure Tanks
Non-pressure tanks are fabricated from mild steel with a minimum
thickness of 6 mm in accordance with A P I standard 12F, for tanks smaller
than 5 m diameter, and A P I standard 12D, for larger size.
Pressure Vessels
Pressure vessels are fabricated in accordance with the ASME Boiler and
Pressure Vessel Code, Section VIII, Rules for Construction of Unfired
Pressure Vessels.
Construction may be based on Division 2 of the Code
which is more rigorous than Division 1, but Division 1 is usually adequate.
Vessels subject to internal pressure are designed for the maximum
operating pressure plus 10% or 2 kg/cm (19 kPa), whichever is greater.
Vessels subject to external pressure are designed to withstand atmospheric pressure.
A minimum corrosion allowance of 2 mm is added to the
thickness of the plate.
calculated
184
Juice Heaters
Standard juice heaters are fabricated of mild steel with stainless steel
2
tubes and designed for working pressures of 6 k g / c m (59 kPa) on the juice
2
side and 2 k g / c m (19 kPa) on the steam side at a maximum juice velocity
of 2 m/sec. Tubes are 38 mm outside diameter made of 18 gauge type 304
stainless steel, annealed and pickled. ASTM-A-249 is a suitable specification. Tube sheets have a minimum thickness of 28.5 mm.
Evaporators
Standard evaporator bodies are fabricated from mild steel with stainless steel tubes and designed for a working pressure of 2 kg/cm (19 kPa).
Tubes are 38 mm outside diameter made of 18 gauge type 304 stainless
steel, annealed and pickled.
ASTM-A-249 is a suitable specification.
Minimum thickness of steel plate is 16 mm for shell, 19 mm for top and
bottom and 28.5 mm for the tube sheets.
One front sight glass is located just above the tube sheet, with a light
source at the back at a height of 1.5 m above the tube sheet.
Vacuum Pans
Vacuum pans are fabricated from mild steel with stainless steel tubes
2
and designed for a working pressure of 2 k g / c m (19 kPa). Tubes are 100
mm outside diameter made of 18 gauge type 304 stainless steel, annealed
and pickled. ASTM-A-249 is a suitable specification.
Minimum thickness
of steel plate is 16 mm for shell, 19 mm for top and bottom and 28.5 mm
for the tube sheets.
Optimum tube length is 800 mm. Striking capacity is a maximum of
1.5 m above the tube sheet. Bottom is stream-flow design.
Front sight glasses provide visibility from the tube sheet to the striking
level, with a light source at the back above the striking level. A 508-mm
diameter man-door is fitted above the tube sheet.
A top-driven mechanical circulator is standard.
PIPING
Process and utility piping is designed in accordance with ANSI-B-31.3
Code for Pressure Piping. General piping is fabricated from schedule 40
ASTM-A-53 black steel pipe. Minimum wall thickness for exhaust, vapor
and vacuum lines is as follows:
Diameter
mm
Wall thickness
mm
400 - 720
720 - 1,150
over 1,150
The minimum corrosion allowance is 1.25 mm.
6
8
10
185
Design standards are for the
velocities.
following maximum allowable stream
Stream
Maximum velocity
m/sec
Exhaust steam
Vapor (pressure)
Vapor (vacuum)
Air
Water
Juice
Syrup
Molasses
Massecuite
30
30
60
30
2
1.5
1
0.5
0.2
Piping, larger than 50 mm diameter, is generally all-welded.
Stainless-steel piping is appropriate for piping subject to rapid corrosion
such as mill juice lines.
Piping for temperatures exceeding 50°C requires insulation.
VALVES
Valve specifications include hydrostatic testing by the manufacturer at
twice their normal working pressure. All valves working at pressures above
10 kg/cm
(98 kPa) are steel-body type. Valves over 40 mm diameter
require flanged connections. Low-pressure flanges are ASTM-A-181 grade
1/11 standard; high-pressure flanges are ASTM-A-105 grade 1/11 standard.
Gate Valves
Gate valves are used for complete shut-off purposes.
They are
commonly used on steam, vapor and water lines; they may be used for
massecuites and other streams.
Valves on steam lines over 200-mm
diameter are fitted with a pressure-equalizing bypass and globe bypass
valve. Valves on exhaust steam and vapor lines over 300 mm diameter are
similarly equipped.
Butterfly Valves
Butterfly valves are all metal construction with an indicator showing
the position of the vane. They are used primarily for control purposes.
When used on exhaust steam and vapor lines over 300 mm diameter, they
are fitted with a pressure-equalizing globe bypass valve.
Diaphragm Valves
Diaphragm valves are fitted with synthetic rubber discs with sealed
bonnets and bleed hole for indication of rupture. They are of general use
for juices, syrup, molasses, milk of lime and caustic soda.
186
Globe Valves
Globe valves are used primarily for flow control but also, should be used
on bypass lines on pressure-reducing valves.
Check Valves
Check valves up to 50 mm diameter are all brass swing type.
over 50 mm are swing type but have iron bodies bronze fitted.
Valves
Pressure Reducing Valves
Pressure reducing valves are made with steel bodies and hardened
stainless steel or stellite discs. They are fitted with a globe bypass valve.
Control Valves
Control valves used for pressure reducing or flow control are provided
with a bypass to permit servicing while the equipment is in operation.
PUMPS
Pumps, in general, conform to A P I Standard 610. Recommended design
capacity is 25% greater than the maximum working capacity. Selection is
made on the basis of pump performance curves, with care, to remain below
the point of maximum efficiency.
Materials of contruction for most uses consist of a cast-iron casing,
bronze impeller and carbon steel shaft. These pumps can be used on water,
clarified juice, syrup and molasses. Pumps for mill juices, mixed juice *nd
muds are designed of corrosion and abrasion-resistant materials //ith
stainless steel shafts.
Glands are water-sealed. Use of mechanical seals is limited.
Pump drives are in accordance with A P I Standard 601.
frame-mounted with flexible couplings.
Drives are
ELECTRICAL
All electrical equipment and material is designed in accordance with
the standards of ANSI and N E M A and designated for continuous operation.
Motors
Standard motors are 440
enclosed type, fan cooled.
V, 60
cycle,
3 phase, induction,
totally
Motors up to 15 kw may have direct, on-line starters. Larger motors
have separately housed control, center-type combination starters with
circuit breaker, magnetic switch and 3-element thermal relay, designed to
operate in the range of 10% overvoltage to 15% undervoltage.
Motors larger than 15 kw are fitted with tubular-type heaters which
will hold the windings higher than ambient temperature but not above the
operating temperature.
187
Low Voltage Switchgear
Switchgear is designed in accordance with ANSI C37.13 and N E M A SG3
standards. Elements are removable and consist of a fuseless 3-pole air
circuit breaker, overcurrent relay, interpole barriers and quenches,
mechanical pushbutton trip and space heater.
GEARS
Gears are designed to ANSI B6 standards.
Gears are designed for
continuous service and to withstand stall conditions.
General service
factors, based upon the power rating of the prime mover are:
Service Factor
Mill, knives and shredder drives
Cane, bagasse and sugar conveyors
Massecuite pumps
Cane feeder table and crystallizers
Fans
Centrifugal pumps and others
2.00
2.00
2.00
1.75
1.75
1.25
TURBINES
Steam turbines are specified according to N E M A SM-20 or A P I 615
standards. Turbines for mill drives are capable of 200% starting torque.
Normal operating speed is set at 80% of maximum speed with a capability
of maintaining full load at 50% of maximum speed.
Normal fittings include the following:
Partial load and overload hand valves.
Woodward oil relay governor with electrically 2-to-l remote speed
changer and hand-operated speed changer for full range at the
turbine.
Independent, overspeed governor with hand-operated trip at
turbine, operating an independent shut-off valve.
the
Relief valve on the exhaust line ahead of the shut-off valve.
Independent lubrication system with integral shaft-driven oil pump
and electrically-driven auxiliary pump with duplicate elements
including low oil-pressure and high oil-temperature alarms, low oilpressure automatic trip, cooler, relief valve and gauges.
Remote control solenoid trip.
- Excess back-pressure automatic trip.
High-pressure and exhaust-steam isolating valves.
Complete instrumentation including tachometer; pressure gauges for
high-pressure steam, exhaust steam and nozzle-pressure; and oilpressure gauge.
188
BOILERS
Boilers are designed to ASME PG and PW standards.
Bagasse-fired boilers are designed to burn bagasse of 50% moisture
content at a boiler efficiency of 68% with 42% excess air. Supplementing
oil burning capability is provided.
Standard furnace equipment includes spreader-stoker bagasse feed, oil
burner, traveling grate, forced- and induced-draught fans, wet scrubber, air
pre-heater and economizer.
A feedwater deaerator and continuous blowdown equipment are necessary.
Boiler control is automatic with safety trips for low water level,
induced draught fan failure, instrument air failure and oil burner flameout.
REFERENCES
1
2
3
4
5
6
American National Standards Institute (ANSI), 1430 Broadway, N e w
York, N Y 10018
American Society of Mechanical Engineers (ASME), 345 East 47th
Street, New York, N Y 10017
American Society for Testing and Materials (ASTM), 1916 Race Street,
Philadelphia, P A 19103
American Petroleum Institute ( A P I ) , 1801 Κ Street, N . W . , Washington,
D . C . 20006
American Welding Society ( A W S ) , 2501 N . W . 7th Street, Miami, FL
33125
National Electrical Manufacturers Association ( N E M A ) , 2101 L Street,
N . W . , Washington, D . C . 20037.
189
SELECTED REFERENCE BOOKS FOR THE TECHNOLOGIST
Introduction to Cane Sugar Technology
G. H. Jenkins, Elsevier,
Amsterdam, 1966, 478 pp.
Cane Sugar Handbook
G. P. Meade ôc J.C.P. Chen,
10th Ed., John Wiley <5c Sons,
New York, 1977, 947 pp.
Handbook of Cane Sugar Engineering
Principles of Sugar Technology
E. Hugot, 2nd Ed. with
collaboration and translation
by G. H. Jenkins, Elsevier,
Amsterdam, 972, 1079 pp.
P. Honig, Elsevier, Amsterdam, Vol. I 1953, 767 pp, Vol.
II 1958, 567 pp, Vol. Ill 963,
711 pp.
The Manufacture of Sugar
from Sugarcane
C. G. M. Perk, Sugar Milling
Research Institute, Durban,
1973, 212 pp.
Manufacture and Refining of
Raw Sugar
V. E. Baikow, 2nd Ed., Elsevier,
Amsterdam, 1982, 588 pp.
Machinery and Equipment of the Cane
Sugar Factory
L. A . Tromp, Norman Roger,
London, 1936, 644 pp.
Cane Sugar
Noel Deerr, 2nd Ed., Norman
Rodger, London, 1921, 644 pp.
Principles of Cane Sugar Manufacture
J. G. Davies, Norman Rodger,
London, 1938, 144 pp.
Technology for Sugar Refinery Workers
Oliver Lyle, 3rd Ed., Chapman
<5c Hall, London, 1957, 663 pp.
Evaporation
A . L. Webre, C. S. Robinson,
The Chemical Catalog, New
York, 1926, 500 pp.
The Efficient Use of Steam
Oliver Lyle, Her Majesty s
Stationery Office, London,
1947, 912 pp.
!
190
Beet-Sugar Technology
R. A . McGinnis, 2nd Ed., Beet
Sugar Development Foundation, Fort Collins, 1971, 835 pp.
By-Products of the Sugar
Cane Industry
J. M. Paturau, 2nd Ed., Elsevier,
Amsterdam, 1982, 366 pp.
Sugar Factory Analytical Control,
Official Methods of the Hawaiian
Sugar Technologists
J. H. Payne, ed., Elsevier,
Amsterdam, 1968, 190 pp.
System of Cane Sugar
Factory Control
J. L. Clayton, ed., 3rd Ed.,
International Society of Sugar
Cane Technologists, Brisbane,
1971, 87 pp.
Sugar Analysis ICUMSA Methods
F. Schneider, ed., ICUMSA,
Peterborough, 1979, 265 pp.
Laboratory Manual for Queensland
Sugar Mills
5th Ed., Bureau of Sugar
Experiment Stations,
Brisbane, 1970, 250 pp.
Laboratory Manual for South African
Sugar Factories
2nd Ed., South African
Sugar Technologists
Association, Durban, 1977.
Basic Calculations for the Cane
Sugar Factory
J. Eisner, Booker Brothers,
McConnell, London, 1958, 24
pp.
Handbook of Cane Sugar Technology
R. B. L. Mathur, Oxford <5c
IBH, New Delhi, 1975, 498 pp.
191
SUBJECT INDEX
A C O N I T ATE
Crystals, calcium magnesium,
114
ANALYSIS
Procedures, 1-8
Schedule, 9-11
ASH
Molasses, 117-119
conductivity, rate of
crystallization effect, 125
exhaustibility effect, 117
rate of crystallization
effect, 118, 119
Sugar, 131
BAGASSE
Analysis, 4
Calorific value
gross, 138; net, 138
Dewatering, 39, 45
Drying, 143
Fiber, 49
Fiber Index, 28
Fines, filter, 64
Fractional Fiber Content, 28
Measuring, 3
Moisture
diffusion, 45
extraction effect, 50
fuel value effect, 51
milling, 49
Operational targets, 171
Pol
diffusion, 41, 59
milling, 50, 51, 55, 59
Rules of thumb, 169
Sampling, 3
Screening, 64,
BOILER, 137-145
Control, 137
Design, 137
Efficiency, 137, 138
Engineering specification, 188
Heat balance, 138, 140
Maintenance, 162
Operation
air supply, 142
bagasse quality, 142
oil firing, 143
stack emission, 143
water 143
Sensible heat loss, table, 139
Water
analysis, 8
blowdown, 142
composition, 140
control, 143
deaeration, 141
heating, 141
sampling, 8
treatment, 140, 141
BOILING POINT E L E V A T I O N , 77,
83
BOILING, SUGAR, PROCEDURES,
88, 102
BOOKS, R E F E R E N C E , 189-190
BUSTER,35
CANE
Cleaning, 15-21
diagram, 16
efficiency, 18
fibrous trash removal, 15, 17
losses, 20
rock, gravel, sand removal, 15
thinning, 15, 17
washing, 15, 17
waste disposal, 20
Composition, 19, 173
absolute juice, 175
fiber, types, 176
fiber bundles, 176
juice types, 175
parenchyma, 176
192
total juice, 175
Deteriorated, 119, 125
crystallization rate effect,
125
crystal elongation effect, 125
Direct analysis, 175
Drought affected, 126
Dry weight produced, 19
Extraneous matter
delivered to factory, 19
diffusion effect, 56
fuel value, 144
milling effect, 54
removal, 15-21
rule of thumb, 169
Field
analysis, core, 2
analysis, direct, 2
extraneous matter, 19
measuring, 1
sampling, 1
Preparation
cell rupture, 24, 43
diffusion, 35
displaceability index, 24,
31, 43
milling, 24
operational targets, 170
shredders, 24
Prepared
analysis, 3
measuring, 3
sampling, 3
Processing, regional variations
clarification, 179
control, 177
diffusion, 178
manpower, 181
milling, 178
steam generation, 180
sugar boiling, 179
Stalk
analysis, 3
cross section, 174
rule of thumb, 169
sampling, 3
structure, 173
Tops
composition, 19
fuel value, 144
percent, 19
rule of thumb, 169
Trash
delivered to factory, 19
diffusion effect, 56
fuel value, 144
milling effect, 54
removal, 15-21
rule of thumb, 169
Varieties, 176
C A N E C L E A N E R , 15-21
carding drum, 15
cascader, 17
collared rolls, 17
combing drums, 17
diagram, 16
efficiency, 18
losses, 20
Olsen rolls, 17
operation, 18
sink-float bath, 15
tumbling conveyor, 15
velocity bath, 15
waste disposal, 20
water reuse, 20
C A N E S A L V A G E R , 18
C A R D I N G D R U M , 15
C A S C A D E R , 17
C E L L R U P T U R E , 23, 24, 31, 43
CENTRIFUGALS
Batch machines, 113-115
Continuous machines, 115, 116
Maintenance, 165
C E N T R I F U G A T I O N , 113-116
Commercial sugar, 113
Low grade sugar, 114, 115
Operation, 115, 116
C I R C U L A T I O N , V A C U U M P A N , 86,
87
CIRCULATOR, MECHANICAL,
V A C U U M P A N , 87, 102
C L A R I F I C A T I O N , 61-70
Bagasse fines, 64
Clarifiers, 68
Filters, rotary vacuum, 64-68
Juice heaters, 63
Liming, 61
Magnesium oxide, 62
pH, 61
Phosphatation, 62
193
Polyelectrolytes, 61
Sulfitation, 63
C L A R I F I E D JUICE
Screening, 70
CLARIFIERS
Flash tank, 68
Holdover juice, 69
Maintenance, 164
Operation, 69
Settling area, 69
Settling space, 69
C L E A N E R , C A N E , 15-20
C O L L A R E D R O L L S , 17
COLOR
Clarification effect, 61
Juice, 61
Sulfitation effect, 63
Sugar, 128, 129
C O M B I N G D R U M S , 17
CONDENSATE
Analysis, 8
Boiler use, 140
Composition, 140
Exhaust, 140
Removal, 75
Sampling, 8
Vegetal, 140
CONTROL
Boiler, 137
Boiler water, 143
Diffuser, 46
Factory, 1
Milling, 32
C O N T R O L L E R S , 157
CONVEYORS
Maintenance, 160
CRYSTAL
Color, 128
Content, 86
Elongation, 119, 125
Growth rate, 107
Measurements, 107
Size, 81, 128
Yield, 106
CRYSTALLIZATION BY C O O L I N G ,
95-99
C R Y S T A L L I Z A T I O N R A T E , 95,
105-107, 118
Ash effect, 118, 119
Conductivity effect, 125
Crystal elongation effect, 119
Crystal growth rate, 107
Deteriorated cane effect, 125
Deposition and diffusion, 95
Purity effect, 106, graph 108
Reducing sugar-ash ratio
effect, 125
Supersaturation effect 105, 107
Temperature effect 105, 107
CRYSTALLIZER, LOW GRADE
Design, 96
Instrumentation, 97
Maintenance, 166
Operation
batch, 97
continuous, 98
continuous, diagram, 99
C R U S H E R , 24
DENSITY
Bagasse, 169
Cane, 169
Sugar, 170
DESSIN F O R M U L A , 78
DETERIORATION FACTOR,
S U G A R , 130
DIFFUSER
Flow balance diagram, 42
Gradients, graph, 38
Juice percent fiber, diagram,
44
Silver ring
design, 36
operation, 41
DIFFUSION, 35-47
Bagasse dewatering, 39
Cane preparation, 35
Comparison with milling, 59
Diffuser gradients, graph, 38
Effect of extraneous matter,
56
Flow balance diagram, 42
Juice displacement, 36
Juice percent fiber, diagram,
44
Maintenance, 161
Operation, 41-47
bagasse dewatering, 45
bed depth, 41
bed permeability, 41
194
cell rupture, 43
control, 46
dilution, 43
diffuser speed, 41
draft, 41, 43
flooding, 45
press return, 43
temperature, 45
Press juice
percolation rate, 40
treatment, 39
Recovery and losses, 59
Silver diffuser
design, 36
operation, 41
DILUTION
Diffusion, 43
Milling, 30, 31
Operational target, 170
D I S P L A C E A B I L I T Y I N D E X , 24, 31,
43
D O U W E S - D E K K E R , formula, 120
D R A F T , 41, 43
D R A I N A G E , 25, 27
DRYING
Bagasse, 143
Sugar, 115
EISNER, 148, 150
ENGINEERING SPECIFICATIONS,
183-188
Boilers, 188
Evaporators, 184
Gears, 187
Juice heaters, 184
Motors, 186
Piping, 184
Pumps, 186
Switchgear, 187
Tanks, 183
Turbines, 187
Vacuum pans, 184
Valves, 185
ENTRAINMENT
Evaporator, 76
Separators, 76
EQUILIBRIUM RELATIVE
H U M I D I T Y , 133
ESCRIBED V O L U M E , 28
E V A P O R A T I O N , 71-80
Multiple effects, 71
Vapor bleeding, 72
EVAPORATOR
Capacity, 73
Calculations, 77-80
Engineering specifications, 184
Maintenance, 165
Operation
control, 74
condensate removal, 75
condenser, 74
entrainment, 76
malfunctions, 77
non-condensible gas, 75
scaling, 75
scale removal, 76
EXPECTED PURITY, FINAL
MOLASSES
Douwes-Dekker formula, 120
T
Hawaiian Sugar Planters
Association formula, 117,
118
Hugot formula, 120
Sugar Research Institute,
Australia, formula, 119
EXHAUSTIBILITY, F I N A L
MOLASSES, 117
Douwes-Dekker formula, 120
Hawaiian Sugar Planters'
Association formula, 117,
118
Hugot formula, 120
McGinnis, chart, 109
Reducing sugar-ash ratio, 117
Sugar Research Institute,
Australia, formula, 119
Viscosity effect, 117-119
E X H A U S T I O N , MASSECUITES,
106, chart 109
EXTRACTION
Bagasse moisture effect, 50
Diffusion, 35, 59, 60
Extraneous matter effect, 55
Milling, 23, 59, 60
EXTRANEOUS MATTER
Diffusion effect, 56
Fuel value, 144
Milling effect, 54
Removal in cleaner, 17-20
Rule of thumb, 169
195
E X T R U S I O N , 29, 31
FALSE G R A I N , 129
FANS
Maintenance, 167
FEEDING DEVICES
Over feed roll, 25
Pressure feeder, 26
Two-roll feeder, 26
Under feed roll, 25
FIBER
Bagasse, 49
calorific value, 138
fiber index, 28
Fractional Fiber Content,
28
Cane, 176
fiber bundles, 176
internode, 173
node, 173
parenchyma, 176
Fuel
leaves, 144
stalk, 144
top, 144
Trash, fibrous, 54
FIBERIZER, 35
FIBER I N D E X , 28
FIBROUS TRASH
Delivered to factory, 19
Diffuser effect, 57
Fuel value, 144
Milling effect, 54
Removal in cleaner, 17
Rule of thumb, 169
FILTER A B I L I T Y , S U G A R , 130
FILTER, R O T A R Y V A C U U M , 64-68
Bagasse fines, 64
Cake
pol, 68
quantity, 68
washing, 67
Maintenance, 165
Mixing fines, with settlings, 67
Operating action diagram, 66
Speed, 67
Station diagram, 65
Vacuum, 67
FILTER C A K E
Analysis, 5
Measuring, 5
Operational target, 171
Pol, 68
Quantity, 68
Rule of thumb, 170
Sampling, 5
Washing, 67
FLASH T A N K , 68
F L A S H I N G , 68
F L O O D I N G , DIFFUSER, 45
F L U E G A S , 138-140, 143
F O R W A R D SLIP, 29, 31
F R A C T I O N A L FIBER C O N T E N T ,
28
FUEL VALUE
Bagasse, 51
Fibrous trash, 144
GAS
Flue, 138-140, 143
Non-condensible, 75
GEARS
Engineering specifications, 187
Maintenance, 164
GENERATORS
Maintenance, 163
G R A I N , FALSE, 129
GROOVING
Chevron, 27
Circumferential, 26
Juice, 27
Messchaert, 27
1
HAWAIIAN SUGAR PLANTERS
ASSOCIATION
Molasses exhaustibility formula,
117, 118
HEAT
Latent, evaporation, table, 149
Sensible, flue gas, table, 140
Utilized, steam, table , 151
HEAT B A L A N C E
Boiler, 137-140
Factory, 80
HEATING SURFACE
Vacuum pan, 87, 102
HUGOT
Final molasses expected
purity formula, 120
HUMIDITY
196
Sugar storage effect, 133
HYDRAULIC LOADING
Mills, 31
IMBIBITION
Effects, 31
Operational target, 170
Quantity, 30
Temperature, 30
INCLUSIONS
Crystal, 128, 129
I N S T R U M E N T A T I O N , 153-158
Controlling, 157
Crystallizer, 97
Indicating, 153
Vacuum pans
commercial, 88
low grade, 102
INSTRUMENTS
Controlling, 157
Indicating
conductivity, 156
consistency, 156
density, 156
flow, 154
level, 155
pH, 156
pressure, 154
thermocouples, 154
thermometers, 153
Maintenance, 167
Operation, 158
INVERSION
Diffusion, 59
Milling, 59
Processing, 61
JUICE
Absolute, 175
analysis, 4
calculated, 4
Clarified
analysis, 5
sampling, 5
screening, 70
Diffusion
percent fiber, diagram, 44
Filtrate
analysis, 5
sampling, 5
First expressed
analysis, 4
sampling, 4
Holdover, 69
Last expressed
analysis, 4
sampling, 4
operational target, 171
Mixed
analysis, 5
measuring, 5
sampling, 5
rule of thumb, 169
Press return
analysis, 5
sampling, 4
treatment, 39
Total in cane, 175
JUICE DENSITY C U R V E S , 32, 33
JUICE D I S P L A C E M E N T , 36
JUICE HEATERS
Engineering specifications, 184
Juice temperature, 63
Juice velocity, 64
Maintenance, 164
Scaling, 64
KNIVES, 23
Maintenance, 160
KRAJEWSKI C R U S H E R , 24
LIMING, 61
LOSSES
Clarification, 121
Diffusion, 59
Filter cake, 68, 171
Inversion, 59, 61
Mechanical, 121
Milling, 59
Molasses, 121, 171
Processing, 121
Sugar crystallization, 121
Undetermined, 121, 171
LOW G R A D E S U G A R
C R Y S T A L L I Z A T I O N , 95-103
Crystallization by cooling, 95
Crystallizer operation, 97
Example, 102
Instrumentation, 102
Procedure, 102
197
Seed slurry calculation, 102
Vacuum pan crystallization, 98
Vacuum pan operation, 100
M A C E R A T I O N , 30, 31, 170
M A G N E S I U M OXIDE, 62
M A G O X , 62
MAINTENANCE, EQUIPMENT,
159-168
Boiler, 162
Centrifugal, 165, 166
Clarifier, 164
Conveyor, 160
Crystallizer, 166
Diffuser, 161
Fan, 167
Filter, 165
Gear reducer, 164
Generator, 163
Heater, 164
Instrument, 167
Knives, 160
Mill, 161
Motor, 163
Piping, 167
Pump, 166
Scales, 164
Shredder, 160
Tank, 167
Turbine, 162
Vacuum pan, 165
Valve, 167
MASSECUITE
A-massecuite
boiling, 81-86, 88
quantity at 85 purity, 90
quantity at 80 purity, 91
seeding, 85
B-massecuite
boiling, 86, 88
quantity at 85 purity, 90
quantity at 80 purity, 91
C-massecuite
boiling, 98-103
quantity at 85 purity, 90
quantity at 80 purity, 91
Analysis, 6
Operational targets, 171
Quantity calculations, 89
Rules of thumb, 170
Sampling, 6
Three-massecuite system,
chart, 110
Viscosity
crystal content effect, 105
purity effect, 105
solids effect, 105
temperature effect, 105
M E S S C H A E R T G R O O V E S , 27
MC GINNIS, MOLASSES
EXHAUSTIBILITY C H A R T , 109
MILL R A T I O , 28
M I L L I N G , 23-34
Comparison with diffusion, 59
Extraneous matter effect, 54
General effects, 31
Mill setting, 28-30
escribed volume, 28
fiber index, 28
fractional fiber content, 28
mill ratio, 28
procedures, 29
reabsorption factor, 29
turner plate, 30
work opening, 28
Operation, 27-33
Power, 31, 32
Recovery and losses, 59
MILLS, 23-25
Maintenance, 161
MOISTURE
Bagasse, 49
extraction effect, 50
fuel effect, 51
Sugar, 130
MOLASSES
A , B, C
analysis, 6
purities, 86
sampling, 6
MOLASSES, F I N A L , 117-120
Analysis, 7
Ash, 117
Decomposition, spontaneous,
135
Drought cane effect, 126
Exhaustibility
ash effect, 117
Douwes-Dekker formula, 120
198
Hawaiian Sugar Planters'
Association formula, 117,
118
Hugot formula, 120
McGinnis chart, 109
reducing sugar-ash ratio, 117
Sugar Research Institute,
Australia, formula, 119
viscosity effect, 117-119
Handling, 134
Measuring, 6
Operational target, 171
Percent pol in syrup, table, 123
Percent 96 D A sugar, 124
Purity-refractometer solids,
1.2 supersaturation, 101
Recirculation, low grade
sugar, 106
Reducing sugar-ash ratio, 117
Rules of thumb, 170
Sampling, 7
Storage, 134, 135
Temperature effect, 135
Viscosity-temperature
relations, 96
MOTORS, E L E C T R I C A L
Engineering specifications, 186
Maintenance, 163
M U L T I P L E EFFECTS,
E V A P O R A T O R , 71
N O N - C O N D E N S I B L E G A S , 75
N O N S U G A R S , 89-92
N U C L E A T I O N , 81, 85
OLSEN R O L L S , 17
O P E N C E L L S , 23, 24, 31, 43
OPENING
Mill, work, 28
Turner plate, 30
O P E R A T I O N A L T A R G E T S , 170-171
Bagasse, 171
Cane preparation, 170
Filter cake, 171
Final molasses, 171
Imbibition, 170
Last expressed juice, 171
Sugar, 171
Syrup, 171
Undetermined loss, 171
P A N D R O P , 86
PAN, VACUUM
Circulation, 86, 87
Design, 87, 102
Heating surface, 87, 102
Instrumentation, 88, 102
Maintenance, 165
Mechanical circulator, 87
PERCOLATION RATE
Press juice, 40
PERK, 105
pH
Boiler water ,141
Clarification, 61
Deteriorated cane, 119
Diffuser, 38, 45
Holdover juice, 70
Molasses, 61
Process streams, 61
Syrup, 68
Unlimed juice, 59
P H O S P H A T A T I O N , 62
PIPING
Engineering specifications, 184
Maintenance, 167
POL
Bagasse
diffusion, 41, 59
mill, 59
targets, 171
Cane, 169, 173
Diffuser juice, 38
Filter cake, 68, 171
Open cells, 23, 24, 31, 43
Sugar, 127, 171
Tops, 169
Undetermined loss, 171
P O L Y E L E C T R O L Y T E S , 61
POWER
Boiler design, 137
Factory requirements, 148
Milling, 31, 32
Relations of steam, 151
P R E P A R A T I O N , C A N E , 24
Diffusion, 35, 43
Displaceability Index, 24, 31, 43
Milling, 24, 31
Open cells, 23, 24, 31, 43
PRESSURE F E E D E R , 26
PUMPS
199
Engineering specifications, 186
Maintenance, 166
PURITY
Cane
juices, 175, 176
sections, 173
Crystallization rate effect, 106
Drop, massecuites, 86-87
Final molasses
continuous centrifugal
increase, 116
exhaustibility, 117
expected, Douwes-Dekker
formula, 120
expected, Hawaiian Sugar
T
Planters Association
formula, 117, 118
expected, Hugot formula,
120
expected, Sugar Research
Institute, Australia
formula, 119
operational target, 171
recirculation, low grade
sugar, 106
rule of thumb, sucrose-pol,
170
Juice
rules of thumb, 169, 170
Low grade sugar
molasses recirculation
effect, 170
Massecuites
A-boiling, 81
B-boiling, 86
C-boiling, 100
operational targets, 171
quantity, 89-91
viscosity effect, 105
Molasses
boiling point-refractometer
solids at 1.3 supersaturation, 84
refractometer solids, 1.2
supersaturation, 101
SJM formula, 121
Syrup
molasses quantity effect,
table, 123
molasses-sugar ratio effect,
124
recovery, effect, table, 122
rule of thumb, 170
QUALITY
Cane, 119
Sugar, 127
R A T E OF C R Y S T A L L I Z A T I O N
Ash effect, 118
Crystal growth, 107
Crystal elongation, 119
Diffusion and deposition, 95
Purity effect, 106; graph, 108
Supersaturation effect, 105
Temperature effect, 105
R E A B S O R P T I O N F A C T O R , 29, 31
R E C I R C U L A T I O N OF MOLASSES
Low grade sugar, 106
R E C O V E R Y F A C T O R S , 121-126
REDUCING SUGAR-ASH RATIO,
117-120
R E F E R E N C E BOOKS, 189-190
R E F R A C T O M E T E R SOLIDS
Diffuser juice, 38
Mill juice curves, 32
Molasses
purity-temperature
relations, 101
REGIONAL VARIATIONS, CANE
PROCESSING, 177-181
Clarification, 179
Control, 177
Diffusion, 178
Manpower, 181
Steam generation and use, 179
Sugar boiling, 179
REPORT
Daily, form, 12
Recovery and Loss, form, 14
Weekly, form, 13
RILLIEUX P R I N C I P L E , 71
R U L E S OF T H U M B
Bagasse, 169
Cane, 169
Fibrous trash, 169
Filter cake, 170
Final molasses, 170
200
Low grade sugar, 170
Massecuites, 170
Mixed juice, 169
Sugar, 170
Syrup, 170
Tops, 169
SAMPLING
Procedures, 1-8
Schedule, 9-11
SCALE, EVAPORATOR
Composition, 75
Removal, 76
SCALES
Maintenance, 164
SCHEDULE, MEASURING, SAMP L I N G , A N A L Y S I S , 9-11
SCREENING
Bagasse, 64
Clarified juice, 70
SEED S L U R R Y
Particle size, 106
Preparation, 106
Quality, 129
Quantity
commercial sugar, 88
low grade sugar, 102
SEEDING
Commercial sugar, 85, 88
Low grade sugar, 100, 102
SETTING, MILLS, 28
SETTLINGS, C L A R I F I E R
Filtering, 64
Mixing fines, 67
SHREDDERS, 24, 35
Displaceability Index, 24, 35
Silver Buster, 35
Silver Fiberizer, 35
Maintenance, 160
SILVER
Buster, 35
Fiberizer, 35
Ring Diffuser, 35, 161
S I N K - F L O A T B A T H , 15
SJM F O R M U L A , 121
SPECIFICATIONS, E N G I N E E R I N G , 183-188
Boilers, 188
Evaporators, 184
Gears, 187
Juice heaters, 184
Motors, 186
Piping, 184
Pumps, 186
Switchgear, 187
Tanks, 183
Turbines, 187
Vacuum pans, 184
Valves, 185
SPEED, MILL R O L L , 31
STEAM
Balance, 148
Power relations, 150
Pressures, 147
Process requirements, 148
Use, 147-152
evaporator effects, 149
evaporator supply, 148
exhaust, 148
high pressure, 148
juice heating, 149
processing, 148
sugar boiling, 150
STEAM G E N E R A T I O N , 137-145
Boilers
control, 137
design, 137
efficiency, 137
operation, 142-143
water, 140-141
STORAGE
Molasses, 134
Sugar, 133
SUCROSE CRYSTALLIZATION
Data, 107
crystal growth rate, 107
single crystal measurements, 107
Generalizations, 105-106
crystal yield, 106
exhaustion, 106
purity effect, 106
supersaturation
effect, 105
temperature effect, 105
viscosity effect, 105
SUGAR
Commercial
analysis, 7
ash, 131
201
caking, 134
centrifuging, 113-115
color, 128
crystal size, 128
crystallization, 81-93
decomposition, spontaneous, 134
deterioration factor, 133
filterability, 130
handling, 133
high pol, 110, 114
measuring, 7
moisture, 130, 133
operational target, 171
pol, 127
quality, 127-131
rule of thumb, 170
sampling, 7
storage, 133-134
washing, centrifugal, 114
Low grade
analysis, 7
centrifuging, 114-116
crystallization, 95-103
recirculation of molasses,
106
sampling, 7
washing, centrifugal, 116
Recovery, 121
Remelt
analysis, 7
sampling, 7
S U G A R BOILING
A-massecuite, 81
B-massecuite, 86
C-massecuite, 98
Example, 88, 102
Instrumentation, 88, 102
Low grade massecuite, 98
Procedures, 88, 102
Three-massecuite system,
chart, 110
SUGAR CRYSTALLIZATION
A-massecuite, 81
B-massecuite, 86
C-massecuite, 95
Commercial sugar, 81-93
crystal content and pan
drops, 86
example, 88
instrumentation, 88
operational target, 171
procedures, 88
quantity, 89-93
rule of thumb, 170
seed slurry calculations,
88-89
vacuum pan 81-88
Low grade, 95-103
crystallization by cooling:,
95
crystallizer operation, 97
example, 102
instrumentation, 97, 102
procedures, 97, 98, 102
quantity, 89-93
rule of thumb, 170
seed slurry calculations,
102
vacuum pan, 98-100
Rate, 95, 105-107, 118
Vacuum pan crystallization,
81-88, 98-100
control, 82-86
boiling point elevation, 83
consistency, 84
electrical conductivity,
83
refractometer solids,
82
S U G A R Q U A L I T Y , 127-131
Ash, 131
Color, crystal, 128
Color, whole, 129
Crystal growth, 128
Crystal size, 128
Crystal uniformity, 128
Filterability, 130
Inclusions, 128
Moisture, 128
Operational targets, 171
Pol, 127
Seed, 129
S U G A R R E C O V E R Y , 121-126
Percent pol in syrup, 122
Purity effect, 121
S U G A R R E S E A R C H INSTITUTE,
AUSTRALIA
202
Expected purity molasses
formula, 119
S U L F I T A T I O N , 63
SUPERSATURATION
Crystallization rate effect, 105
Purity, refractometer solids,
saturated, 82
Purity, refractometer solids,
1.2 supersaturation, 83
Purity, refractometer solids,
1.3 supersaturation, 84
Sensing
boiling point elevation, 83
consistency, 84
electrical conductivity, 83
refractometer solids, 82
SWITCHGEAR
Engineering specifications, 187
SYRUP
Analysis, 6
Operational target, 171
Refractometer solids, 71, 74
Rule of thumb, 170
Sampling, 6
T A N D E M , M I L L I N G , 23
TANKS
Engineering specifications, 183
Maintenance, 167
T A R G E T S , O P E R A T I O N A L , 170-171
TOPS, C A N E
Composition, 19
Fuel value, 144
Percent of cane, 19
Rule of thumb, 169
T R A S H , 53-57
Fibrous
delivered to factory, 19
diffusion, effect, 57
fuel value, 144
milling effect, 54
removal in cleaner, 17-20
rule of thumb, 169
Diffuser effect, 56-57
Milling effect 54-56
Removal in cleaner, 17-20
Rule of thumb, 169
T U M B L I N G C O N V E Y O R , 15
TURBINES
Engineering specifications, 187
Maintenance, 162
T U R N E R P L A T E , 30
U N D E T E R M I N E D LOSS
Operational target, 171
V A C U U M FILTERS, 64-68
Bagasse fines, 64
Cake
pol, 68
quantity, 68
washing, 67
Mixing fines with settlings, 67
Maintenance, 165
Operating action diagram, 66
Speed, 67
Station diagram, 65
Vacuum, 67
VACUUM PANS
Circulation, 86, 87
Design
commercial sugar, 87
low grade sugar, 102
Engineering specifications, 184
Heating surface, 87, 102
Instrumentation, 88, 102
Maintenance, 165
Mechanical circulator, 87, 102
Procedures, 88, 102
VALVES
Engineering specifications, 185
Maintenance, 167
V E L O C I T Y B A T H , 15
VISCOSITY
Final molasses
exhaustibility effect,
117-119
purity effect, 105
rule of thumb, 170
temperature relations, 96,
105
Massecuites
crystal effect, 105
purity effect, 105
solids effect, 105
temperature effect, 105
WATER
Boiler
analysis, 8
blowdown, 142
203
composition, 140
control, 143
deaeration, 141
heating, 141
sampling, 8
treatment, 140, 141
Cane cleaner, 20
Condensate
analysis, 8
composition, 140
exhaust, 140
removal, 175
sampling, 8
vegetal, 140
Condenser
analysis, 8
sampling, 8
Drain
analysis, 8
sampling, 8
Imbibition
effects, 31
operational target, 170
quantity, 30
temperature, 30
Maceration, 30
WORK O P E N I N G S , 28